My dilemma is that I have a 5750 and I dont know whether to buy a 6770 and crossfire it with my 5750 or buy a 7770 and use it as a single card. Moneys tight so Im really not looking for anything over the x7xx series. If I would be able to crossfire a 7770 with a 5750, that would also be an option.
Depends on your motherboard and your power supply. Pretty sure you can't combine 7770 with 5750. I would go with a 7770, consumes less power than two cards and less noisy, less heat, more space left in case etc.
since the 6770 is a rebranded 5770, it can be CF with a 5750. the 7770 is a 28nm process as opposed to the 6770/5770 40nm. so cf a 7770 with the 5750 is out of the question http://sites.amd.com/PublishingImages/Public/Graphic_Il...
the 7770 is $130-140, while the cheapest 6770 is $110. CF 6770 would have similar performance to a 6950, so a cf 6770/5750 would have performance similar to (please correct me if I'm wrong) a 6850/6870 (?). a 7770 performance is pretty close to a 6850. In that sense, I would get the 7770.