If your option does not include getting a PC, or at least upgrading your PC (you can get a great video card as cAPSLOC mentioned), then 360 obviously. As for which model... I need to own one to give an answer. Frankly, I find the PC to be far superior to any console in FPS, it's cheaper both in terms of online fees, selection, and game pricing*.
P.S. You need to pay for the 360 online multiplayer service though, so I am not sure if you need to consider that into your budget
P.P.S. In addition, since you have a good PC already, and I assume you play FPS on that, it'll take some getting use to the 360 controller. (I personally prefer mouse over dual analog, but that's just me)
P.P.P.S. Stick with your PC if all you want to play is FPS game, most of the FPS games on 360 will eventually end up PC anyway, it's only a matter of time.
* purely speaking in terms of NA, since it costs about $50 for a PC and Wii game, and $60 for a 360 and PS3 game.
I am a PC gamer, and also have a NDS now, but without access to a Nintendo Wii until End of 2007.
Currently playing: Super Smash Brothers Brawl(Wii), Mystery Dungeon: Shiren the Wanderer(DS), Dragon Quest Heroes: Rocket Slime (DS), WiiFit(Wii)
Games Recently Beaten: Final Fantasy Crystal Chronicles: My Life as a King (Normal; Very Hard after the next DLCs become available)
1 word: RTFA
