about the only thing I've used my xbox for are sports games, and fighters although I have also played Call of Duty 4, and I kinda wish I had gotten Fallout 3 for it as well instead of on the PC, for some reason it keeps crashing on me even though I've tried everything I can think of, including the FFDshow fix.
L4D on PC though all the way. It doesnt require the latest and greatest since it is a source game, but you'll want a decent system.
Also, we need to be able to put Steam names in our profiles as well. Hell we have Xbox 360 tags, why not Steam?