So I was curious as to what the gain for me would be if I upgraded from my crossfired HD 4670's to either a 5770 or a 5850. I only have a 20" monitor and play my games at 1600 X 900 resolution. Also its probably worth mentioning that my motherboard has dells modified 650i chipset so while i do have two x16 PCIe slots they both have only 8 lanes.
A 5770 would be about a 15% increase over your crossfire 4670s and should handle 1600x900 resolution with max settings for awhile. the 5750 crossfire in that list represents the performance of a 5850.
Id say 5770 now if money is short and crossfire another later when things get bogged down, having two 8x pcie lanes wont impact performance. if not then the 5850 will last you mostlikely till the beginning of the 7000 series.
I have been using crossfire on this chipset for about 8 months now. Ever since I updated to the new display drivers from the ati website one of my cards is now read as "disabled" I haven't been able to get it working thus far after going back in forth between drivers and even re-installing windows. So I will patiently wait for new drivers but if they dont come soon (i'm not very patient) I might just start looking for a new card.