Maybe Consider getting an X3 720 ( about 75$ for a used on Ebay, Shipped) and ulcoking the fourth core. You could use the saved 100$ to get an 5850 wich is a much nicer card. If you don't want the 5850/X3 720 go for an 4890>5770 at the same price. I picked up a 4870 1gb for 93$ shipped on ebay, the 4890 are about 150$.
What games are you interested in playing? The 5770 is a weak card, you are paying mostly for the dx11 and eyeinfinity...which that card can't maintain at higher resolutions. It has trouble playing Dirt2 at playable fps at high resolution. 5850 does cost more but....hey, you get what you pay for. 5850 is an awesome card
Yeh, you will end paying extra for a low end 5xxx, buecuase AMD controlls the DX 11 market. The thing is the 5750 can't even do eyefinity very well, so you're paying for nothing. just go with X3 720 Be like i said earlier. Also consider getting a 5850, or if you want to svae 100$, an 4890. Or a 4870 x2.
the 5850 sometimes get upwards of 30% more performance than the 4890....but on some games the difference is less than 15%. Check those benchmarks out cuz if you are playing those games that are getting much more performance gains with a 5850 i'd reccomend getting the 58.....but, we all know what crossfire 4890's are sexy and thats only 400 bucks, 100 bucks more than one 5850 but much more performance than that one 5850.