You won't feel a performance boost if you buy RADEON 5770.
I suggest you go for 5850.
If you don't have the money for that,
I suggest you wait sometime.
Make sure your motherboard supports CrossFire and buy two radeon 5770.
It will give you performance close to or on-par with 5870.
Think before wasting your money on single 5770.
Go for CrossFire 5770.