What do I need to transform my network from Ethernet to WiFi?

Firstly DON'T put your servers on a wi-fi network, but given you only have 12 client machines and based on the assumption that they're all within the range of a single wi-fi base-station then you should be able to just buy a regular wi-fi base station and 12 wi-fi adapter card for your clients.

Plug the base-station into your existing wired network, give it an IP address and set a DHCP range, set the wireless security to WPA2-mode, set a long password, then fit the client cards and enter the same password on them. That should work fine, in fact if your network is only 10Mbps ethernet you could see a performance improvement. Wired is generally safer than wireless and there are safer ways than using WPA2 but it's a good combination of pretty-safe and very easy to setup.

If these are workstations, actual desktop workstations, in the long run using wiring is still probably better. Wireless is useful for visiting users and people who actually use laptops as laptops and not completely as desktops (never completely understood that mentality, personally).

Wired networks provide more security, the bandwidth isn't shared, and tends to be more reliable. Wireless can be hacked more easily (generally), can be interfered with more readily by inadvertent signals from other devices, and there are a number of "whoops" moments that I've run into supporting people (why doesn't this work? Well...is your wireless physically on, is your wireless card seeking that SSID, is it getting proper signal strength, is anything interfering, did you get in the correct password if it's encrypted, etc.)

If you go wireless you first should not go completely wireless for your servers. Bad idea there.

Second you might want to keep some wires for speed or options down the road. You don't know what's going to happen with the change until you test the switchover.

Third depending on how much you rely on your income from your business being reliable, you might want to invest in equipment from Cisco with a service contract. They have high end features which add layers of security and VLAN ability (with the right models) as well as monitoring and management abilities.

Is speed good? Depends on your usage. You don't elaborate on what you're pulling from the servers and what your usage patterns are. If it's relatively low traffic, sure. It won't match what a gigabit switch will deliver over Cat6 or Cat5e cable and never will. If you're all pulling Internet radio streaming, Youtube, editing large files or copying large amounts of data, etc...probably not so much a good idea.

Audit your capacity and usage. You might even get away with running a hybrid wired/wireless network where your heavier users can be wired and your mobile users use the wireless.

Other notes...hmm....WEP is useless if you're securing the network...if it's a small business and you want security you'll need at least WPA encryption...depending on your paranoidometer, you can hide the SSID and use MAC filtering to lock who can and cannot use your network (neither of these will make it foolproof if someone targets you for attack). You'll probably be thankful if you still have some drops for configuring or troubleshooting with a wired connection. Home/SOHO wireless routers, on the cheap, tend to fail anywhere from 3 months to 2 years after purchase, with some really weird consequences (My home network I eventually switched to a dedicated wireless AP after having switch/AP combos that would mysteriously act up and rebooting them caused my entire network to go dark while it restarted, other times I've had wireless units die while the switch worked fine, etc.-save yourself headaches. Combo wireless/switch/routers? avoid them. Get a wireless AP or controller, just for wireless.)

You're going to have more fun with SOHO units that cause weirdness until they're restarted. No rhyme or reason. Spend the $$ on a good Cisco unit with support. Especially for business. If you're doing this to save money, doing it right will still be expensive.

Don't hide the unit in your ceilings or closet if you have animal issues. Trust me. Rats and guano are not pleasant to find all over your WAP (don't ask).

Power over ethernet is your friend if you need to spread out connectivity to remote areas in your business (warehouse? interference issues? Need to place units to give good distribution)

You need to look at your situation and map accordingly. Don't put in multiple units as multiple units. Get units that can bridge (like Airport base stations from Apple) or use range extenders to do it on the cheap.

These all depend on your business needs. Don't whine about cost if you're relying on having your workstations actually working when called upon. If it's truly important, your network and infrastructure is something to invest in, not patch and cobble together.

If you can stand occasional frustrations and headaches and everyone is located more or less closely together, get a SOHO combination unit. I find them to be like the soldering irons found at "the shack". Cheap, disposable, and a source of great catharsis when I throw them out after they screw up my network after a year, and I usually ended up buying 2 so I'd have a spare when the first failed. Until I bought the dedicated AP and a dedicated small switch for my home network. Separating the tasks helped reliability greatly.

Keep in mind that if switching to wireless for small businesses was an instant money saver everyone would have done it and all the pop-computer mags would be crowing about it. As it stands the articles are almost always for homes as it saves home users from having to run cabling and they can surf for...movies...from their couch or chair or wherever they sling their netbook. Businesses are wary of switching entirely to wireless for a reason. They run a business, they rely on their people working and not having to troubleshoot gremlins, and doing it "right" will cost money that yields more convenience but less security and reliability (if not done right!) than wired. And less speed than a dedicates switched environment.

It's not difficult to do - I'd look at "professional" level access points rather than home grade ones even for a small network like this.

If your wired network is poor then its possible that you could see an improvement with wireless with a couple of caveats:

You'd want to be using 802.11n which means base station and workstations need to be able to work with this standard. It isn't a magic "fix all" for wireless network but its generally pretty good.

If you're plugging your access point(s) into a really poor wired network then the wired network is still going to drag down performance.

In any case, I'd strongly advise keeping the servers on a wired connection.

If you have a good wired setup that works well and which you are happy with, I'd be asking what exactly you hope to achieve here, the only ability you'll gain by moving from a good wired network to wireless is mobility, and while that might be reason enough for some people it isn't for everyone.

Something to be aware of, and that many people do not consider, is that when a wireless cell has mixed 802.11 b/g clients, performance will be adversely impacted. When even one 802.11 b client is introduced, the wireless access point will enable a protection mechanism called CTS/RTS to prevent collisions caused by the fact that b clients cannot decode 802.11 g OFDM transmission management/control packets. Also, in a mixed network, g clients must adopt the longer 802.11 b backoff times.

Generally speaking, an 802.11 g-only network is over two times faster than a mixed b/g network.

See:

http://www.cisco.com/en/US/products/hw/wireless/ps4570/products_white_paper09186a00801d61a3.shtml