I'd always go for Nvidia over ATI any day - and before anyone asks, I have tried both!
Realistically you don't require 2GB of graphics memory unless you're running incredibly high resolutions! I'd rather have 1GB of faster graphics memory and processor, rather than 2GB of slower graphics memory.
You will probably get a Molex->PCI-E adapter with the new GPU anyway, I think the 7770 only wants one 6-pin PEG connector doesn't it? You should only need to upgrade if your PSU isn't powerful enough, connectors should be fine.
Originally Posted by LeMarchand
If your stuck for stuff like this - I normally check out Tomshardware and look for the latest comparison run.
They tend to break to into price brackets - decide on your budget and check out the sort of thing you get for your £'s.
Might not give you "the one for you" but might point you in the right sort of direction.