If you main purpose is gaming the Athlon II X3 425 (cheaper) or Athlon II X3 440/445 will be all you really need for a single graphics card.
However with higher-end cards like the 5850 and higher a better CPU can make a difference (especially with CF/SLI), and a Phenom II 955 would be plenty (the lower-clocked ones are fine too).
Personally I say budget is something you need to figure out before AMD/Intel. I find in the low-mid end AMD offers better price/performance for many tasks, but in the higher end Intel holds the performance crown. Right now the Phenom II 945 is probably the single best gaming processor below $150, but the I3 isn't far behind it from Intel.
They are recommending you ask for new system build advice in the Home Build Systems forum
There is a Sticky Topic at the top "READ BEFORE POSTING: Buyer's Guides and Troubleshooting"
and it has a "How To Ask For New Build Advice" that leads you through setting up a request for assistance.
You can ask the same questions there, just in the context of an over all system build, keeping your budget goal and what you want in mind.