Okay first heres the thing: 2x 5770 = 1X 5870, which makes it $100 cheaper. However, the 5750 is not an upgrade over your 4870 as a single card. The 5770 performs on par with the 4870. What would you rather have higher performance or a single card. That question is up to you. If it's a single card go for the 5830 and above. If its two cards outperforming a mainstream single card go for 2x 5770s.
i don't know about 5830. i mean if someone's going for 58xx series he'd go for 5850 because 5830 isn't good for this series people would rather go for a 5770 for a better price.
from the other side: i bought a 9500gt for physx and made it work with 4870 with modded drivers if i wanna go for 57xx series (crossfire) it means no more slot for 9500gt and the money has gone to waste.