Difference between these 3 models? XFX 5770s

The far right one is the original spec release with a full shroud so the hot air is exhausted through the back of the case but restricts some airflow around the card itself so will run slightly hotter itself while keeping case temps a bit lower

far left is 2nd version with a refined cooler and more airflow around the card so will run cooler but will raise case temps a bit so works better in a good case with good airflow

middle card is single slot and newest model (PCI-e 2.1 vs PCI-e 2.0) - haven't really seen much feedback on this version but according to XFX :

It sports a uniquely designed fansink, with a 15% greater total
heatpipe surface area than the HD 5770 dual slot. This single slot fansink is also up to 23.5 dB quieter
than the HD 5770 dual slot. In addition, dust levels are kept down to a minimum with IP-5X dust
protection standard.

Though with the 6850 performing much better for only $40 - $50 more I'd probably go with it over any of the 5770 models unless I was adding a second to an existing system ! (since it has the 256bit memory bus and performs much better)