This article is a good one to start with . You can always model your build based on this $500 build . You could always swap some components in this build to the ones you prefer as long as you find it compatible .
An AMD Phenom X4 955 BE would beat the crap out of the AMD six core processors when it comes to gaming .
I've also heard that the phenom X4 overclocks like crazy even without a good tower cooler .
You could go for a HD6950 over a HD6870 if you find one in the $200 region , by this way you could always eliminate the hassles associated with Xfiring HD68XX cards . The HD69XX cards crossfire scaling are damn good and that is evident from the recent Tom's article on BF3 .
As for the hard disks its kinda a difficult to suggest one as the prices have sky rocketed and its a damn shame !!! Nothing we could do about that . I'll leave it for the veterans in here as to whether it makes sense to get a SSD at this point .
I almost forgot , you need to go for a better mobo if you plan to crossfire or SLI better.