Conack:
I upgraded a friend of mine's HD5770 last week with a MSI HD6950 2GB, and it basically allowed him to go from low detail 1024x768 to high detail 1920x1080 with a higher framerate in BF3!
Then again, he is using an overclocked i5 750 @ 3.4GHz.
So Wesley has a very valid point, that your CPU is perhaps too slow. Could you tell us what game you're playing that is so slow and what CPU & RAM you've got?
Lastly, rather create your own thread, than to derail this one!
Wesley:
Are you sure the framerate would be acceptable with those HD6770/GTX550Ti cards in BF3 at 1080p?! I suppose you would be able to get 30fps @ 1080p (on high detail), which is barely playable.
The HD6950 is quite a bit better than those cards, but then it also costs 2.5x more.
Blackmist:
At 1600x900 without AA, I suppose they (HD6770/GTX550Ti) would do just fine for BF3, however your CPU and RAM might bottleneck the performance in BF3, because BF3 is quite a CPU intensive game! I'm not entirely sure how much RAM BF3 needs, but it shouldn't permanently affect the framerate, but it could definitely make the game stutter every now and then. So perhaps just close as much applications as possible before launching BF3 and close the browser too once BF3 has launched.
Using the 2x molex to 1x 6-pin PCI-E should be used as last resort, but it should work just fine in most cases

If you're going this route, try to use it on a cable that doesn't have a HDD/fan connected to it. If you have lots of spare power cables, use 2 different cables to connect to the 2x molex adapter - because the cables might be on separate PSU rails (but this is unlikely with dual rail PSU's).