If it does what you need it to than why replace it, you still have the option to crossfire the 5770 later if you need to. If you wanted to replace it the lowest I'd pick is the HD 6950 (it wouldn't be worth your time to buy anything lower than this). But again the way to go is the i5 2500k and 8Gb of ram since 4 seems to be the new minimum. You could stick with the corsair 650w you stated originally since it will have enough juice for dual 5770's.
Be sure to post up what you end up picking