What gpu will be able to play farcry 3 on ultra that has a descent price?
It really depends on your budget. I'd say the best bang for your buck card right now is the Radeon HD 7870 though.
im trying to go for a 500-600 build
You're going to be able to play Far Cry 3 on ultra with a $600 build, assuming that the game is as demanding as I think it is...
I'd argue a 7850 is better for the money, mostly because it's a great overclocker. Buy one with a nice cooler for around $200 and overclock it past 7870 stock levels of performance.
the 7950boost is a whole lot of bang, but it might be out of your reach
True, but the performance difference at stock is pretty large from the 7870. Drop down from the 7870 to the 7850 and framerates are almost cut in half, and the 7870 is only around $30 more.
You could do very well with a GTX 480. You can find them as cheap as $175.00. A steal for a 384 bit card.
$50 savings if you go for a $199 7850 like the Sapphire 2gb.