Personally, I use a linksys wrt54g router flashed with ddwrt as a wireless NIC. I use it as a client bridge, so that one office (with multiple wired devices) can talk to my other router. It's excellent because it lets you adjust the transmit power a lot higher than you normally can. Using a different linksys WNIC, I saw around 5 SSIDs when I scan for networks near home. With the DDWRT router/NIC, I see 50!
Then, click on the link for the model. On the page, it might have separate instructions. But basically, if you were to do the steps of most routers, it involves flashing the firmware to a generic version of ddwrt, then flashing with the model specific version.