Set up wireless networking solution for small office (<30 people)?

I have one wireless router (Linksys E2500) that's currently providing wifi for ~18 people on laptops, plus some of their devices (e.g. tablets). The network has gotten slow lately, and we're pretty sure it's due to wireless congestion as plugging in directly to the router eliminates all latency issues.

We have a few options as we see it:

• Upgrade the router to something more robust/industrial
• Get a separate wireless access point, plug it into the router, and instruct some of the people to connect directly to that
• Wire the office with ethernet cable along the walls (this is less desirable, especially since we aren't sure how much longer we're going to be in this space)

What would you recommend?

One thing we already tried is disabling the 2.4ghz spectrum, so only 5ghz remained, but then our Windows users were unable to see the network's SSID.

EDIT: So what we ended up doing was getting a new, beefier router (the E4200) and plugging the internet connection directly into that. Then we plugged the older router (the E2500) into one of the ports on the E4200, changed the gateway, and put it up with a different SSID. Now half the people in our office connect to the old one, and half connect to the new one, and our connection issues are resolved.. for now.

Before you go buying new equipment check the air waves around you. If you are on the wrong channel the frequency you are using just might be saturated. Channels 1, 6, and 11 have the least amount of over frequency overlap, so choose one of them if one is open and available.