Do we need a DNS server when we use OpenDNS?
Are you using Active Directory ? If so you can't really use an external DNS server for internal users. Also it would be more beneficial for us if you update the question with specifics where each roles go and how you want to split it.
Basically if your environment works with AD you can't point your workstations to external dns servers as the workstations need to know how to find your AD controller, how to find each other, how to find other servers. Considering that you do have 40 workstations I presume you must be using Active Directory (otherwise it would be waste of resources not to).
Actually if you do have Active Directory on your server you can't set it up without DNS so if you're splitting roles up it should be something like (bare in mind I don't have knowledge what you have in your network):
- Server 1 - AD/DNS
- Server 2 - SharePoint
- Server 3 - SQL
- Server 4 - Exchange
- Server 5 - 2nd AD/DNS -> if it's different physical machine, otherwise it won't matter as much as when the physical machine is down whole AD/DNS is down.
EDIT: considering your update
- Server 1 - AD/DNS (primary)
- Server 2 - FileServer
- Server 3 - AD/DNS (secondary) - different physical server
Also since you seem to have some free space you could put SharePoint Foundation 2010 in your network and have nice little place for your users to share their work. It's really useful even in such small places. I do have similar setup and having Sharepoint as a central store for files is far better then simple FileServer (of course Fileserver has it's goodies - but SharePoint is good for working on documents - history of changes etc)
I would certainly recommend keeping DNS on an internal server; though with it's a lightweight and very stable role and with a network of your size, It's certainly not necessary to reserve an entire server for it.
The main issue here, is that you'll also need a DNS server that supports Dyanmic Updates for your internal network, since a lot of Active Directory requires it. (If you want to do this with a DNS that isn't Windows' own, then be prepared for a lot of pain.)
I suggest turning off the router DNS, enabling the AD-integrated DNS role on two of your servers (This will be full master/master, so you won't need to worry about replication) and ensuring both servers are passed to the clients by your DHCP server. You can still forward referrals to OpenDNS for the rest of the world.
This is probably the easiest configuration route and also gives your network an extra bit of resilience.
There is also a strong argument for having a local DNS server on the performance side. The DNS server caches lookups, and as users tend to hit the same FQDN often you can cut the connection time down significantly. As little as 50ms slower connections will start degrading the perceived performance of the network. OpenDNS is (in my experience) relatively fast, but there is little they can do about the transmission delay from their server to your network.
So, even if you use a service such as OpenDNS, having a local server to cache lookups gives you benefits.