For an Athlon II X2 2.8GHZ, a Radeon HD 6850 is the most powerful card you should get. Any more than that and you would be better off buying a new PC. For most HD6850s, you should get a good 500W or greater power supply (from a brand name like Corsair, or Seasonic or XFX or Enermax) with a 6 pin PCIe power connector.
With a 6850, you can play diablo 3 on ultra detail at 1920x1080 at 72fps (source: http://www.tomshardware.com/reviews/diablo-iii-performance-benchmark,3195-5.html)
You should also be able to max out starcraft 2 at 1920x1080, ultra detail at 38fps (source: http://www.tomshardware.com/reviews/radeon-hd-6870-radeon-hd-6850-barts,2776-14.html).
For other more demanding games like BF3, you will need to lower the detail to medium in order to get close to 40 to 50 fps.