That will do about as well as a 5870 (~$300 too) performance is always about equal to price, you DO in fact get what you pay for. Except with CPUs and marked-up flagship products like the 480 I stupidly bought but love.
EDIT:I had this setup at one point. (Trifire 5770s, NO UPGRADE PATH EITHER! HOT!)
3 are amazing.
OP do eeeeet