Well my specs are
Core i5 3570k
Asrock Z77 Extreme 4 Mobo
2TB WD Green
Seasonic 750watts Power supply
NZXT Phantom 410 case
nice. so no problem running any card on the planet
first, i would like to state that i would be apprehensive at getting a card with 192 memory bus that costs more than $220. so i am not going to recommend the 660ti. that is my personal philosophy, right or wrong.
looking quickly at a few reviews; according to these benchmarks
the sapphire 7870 OC does hang out of the box with the sapphire 7950 OC; making it a good buy for price/performance.
and according to these benchmarks
the sapphire, when overclocked a little further (1100/7360) can surpass the XFX DD 7970; making it the best performing card IMHO.
so, if you want the more powerful; get the 7950 and overclock it. though i must also state that mileage will vary when overclocking but if you only get (925/5500) it will still hang with any of the big boys.