It depends on the games that you play but since your resolution is low,then your card won't run at its full potential and will be bottle-necked by your CPU.If you already haven't bought the card,then consider something else like GTS 250,its slower but its enough for your resolution.(GTX 460 is overkill for your resolution IMO)
Just get the 460. So long as FPS doesnt drop below 40, you're OK. The 460 will have enough juice left over tof heavier games like Crysis and Metro. Besides, that CPU is more than enough. Just bump up AA.
Dont worry. I'm running a 4870 at 1024*768. My cpu is maybe 10-20% faster core for core than yours. Have no problems in any game- 100+ in everything except the PC killers (ie Crysis, Metro 2033 etc) My 4870 should be about 80% or so as fast as the 460, so no problems there. Full rig in sig.