If you think you'll get a larger monitor, I'd suggest a HD6870 or GTX560. If not, nothing more than a HD6850. You could use a HD6770 or HD6790, but I suspect the price difference over the HD6850 isn't worth it.
Your EA-500D will have no trouble with any of those.
Thanks, I'm going to get Sapphire HD6870 from amazon. Argh, I don't want Dirt3
, NFS all the way..