xfiring the 6770's will be about as powerful as a single 6950. up to you but sounds to me the xifre setup would be your best bet and cheaper allowing for your RAM upgrade you want
FYI 1333 works fine.
you wont need 1600 for a bigger card. or 8gb. I would just add 4 more gb of the 1333 and call it a day save even more money just make sure the timings and such are compatible
Thanks for that I'll give it a try with the 4GB 1333 and the 2x6770's with the correct driver it will an should work.