If the rest of your system is good, you can get High settings on BF2 and HL2. I have a Radeon 9800Pro and just about everything in Half Life 2 is on high for me and it runs great except a couple maps lag me, but luckily they are maps I dont like. BF2 will lag you if the rest of your system cant handle it.
You may not be able to go max on the AA or AF like a 7900GT/X can, but you'll be getting better FPS than a 6800GT.