sounds like a battle between tn panels (120hz) and ips panels (60hz) here...
sure you might get faster refresh rates on your tn panel but unless you are pushing 100+fps it doesnt matter. also consider that tn panels have the worst viewing angles of any monitor and only so-so color accuracy. ips panels have a very wide viewing angle and (not e-ips) have better color accuracy.
as for future proofing, having a large screen reduces the amount of time between upgrades. for longer time between builds stay at 1080p.
i'm running a gtx470 and i'm pushing over 60 on all games i have. this includes crysis, supcom2, starcraft2 and the like. i use a 1080p screen.
for 2560x1600 you need to be packing a more serious graphics card (or multiples) which i do not want to budget for.