In an office of ~15-17 people, we upgraded our old router to a Cisco Small Business RV 220W Wireless-N to handle the load of roughly 18-24 devices online at all times smoothly. The major problem with this router (and apparently, within reason, Cisco's entire above-consumer range) is that it is not dual band.
We're running the router in N-only mode, which is fine, all of the equipment can handle that just fine. However, we have ro run it at only 2.4GHz, which is absurd given the 5GHz spectrum is, as I understand it, one of the biggest avantages N wireless has over G. We have to have it on 2.4GHz as things stand, because iPhones can't connect to 5GHz N networks - only 2.4 (based on our trials). We're all on iPhones so that's problematic.
I've confirmed with Cisco that the router (as their others anywhere near our price range are) are incapable of dual-band operation, as mentioned above, which seems insane to me but there you have it.
The only solution I can dream up is switching the Cisco router into 5GHz broadcast mode, and using an access point to broadcast the signal in 2.4GHz simultaneously. I'm wondering if that alone is at all possible, and then whether a Linksys WRT54GL is capable of handling that (given DD-WRT or another firmware option).