resolution is irrelevant for gpu selection
will a 10+ year old game work at x resolution at the same fps as a new modern game? HELL NO
erm... not true. rez is 1 of the most relivant things to check off when your selecting a gpu. after all you dont wanna try to run a 1920/1080 screen off a 128mb-256mb card/igp.. you need 512mb minimum just for the screen size alone. so it wont matter how powerful your card is if it doesnt have the memory it wont run properly at the rez he requires.
you select your card on certain criteria and screen size is 1 of them if your gonna run 3 screens you want a minimum of 2 gigs preferably 3 as more memory tends to help with higher settings as well. also its pointless running a 3gig gtz 680 on a 1024/768 monitor as it will cause a cpu bottleneck because of runaway fps or low gpu usage due tot he screen not requiring all the ram so you never get the true potential of the card. which would be a waste of money...
so yeah rez is relevant...
op your looking at 200bux minimum to run the game at high on single player and 350 to run at high on multiplayer. multiplayer will force low settings to get a solid 60fps on gulf of oman with a $200 card if you want a steady 60 fps on high settings your looking at gtx 570 or 6950 which are 300+ (maybe look second hand for a 570 they can be had for a lot less than retail if you find the rite seller like on overclockers forum). a m8 just picked up a 580 frozer 2 for £200/$300ish