I have heard that the USB NIC's are just as fast and reliable as the internals. I've never used one tho, so i couldn't tell you. Is your USB adapter 2.4ghz or 5ghz? That will also make a difference if you want a router with 5ghz.

Draft N routers use 2.4ghz channels and 5ghz channels. 2.4ghz is the older standard, and 5ghz band router will still support 2.4ghz networks (enabled thru the router admin options). 5ghz bands are needed for streaming large files over your network; 2.4ghz will work fine for gaming. Also, you should watch out for supposed "dual band" routers; that is, a router that is capable of using either 2.4ghz or 5ghz. Simultaneous dual band router are capable of running 2 networks at the same time (at various bandwidths if the router supports it). Running a higher frequency (i.e. 5ghz) does not mean that your signal will go farther. Actually, it might hamper the distance that you can push your signal, especially if you have multiple walls to pass thru or a lot of electronic interference in your path.

For my network i currently use an Apple Airport Extreme. Simultaneous dual band 5ghz channels, one for my internet/computer gaming and the other for my media server.