Graphics Upgrade

I have:

AMD Athlon 7850 Dual Core (AM2+ 940)
ECS GF8200a Black Edition (GeForce 8000 series)
ASUS EN8500GT 256MB 128-bit GDDR3 PCI Express x16
4GB PNY DDR2

I'm interested in upgrading everything eventually but for the moment I want to focus on a Video Card, the 7770 looks decent but not powerful enough for AA so maybe I'd like to go with the 7870. My current monitor maxes out at 1080p apparently.

My question is how much of a performance loss will there be with this older rig? And is there a site somewhere that can show performance stats with various component configs? I tried the Futuremark Hardware Channel beta but it doesn't have the older hardware. Am interested in playing BF3 and GW2.