If you're just using of for gaming purposes, the FX-57 is the way to go. When it comes to gaming, the games don't utilize the two cores, so it doesn't make a big difference when it comes to gaming.
I heard FEAR has multithread though...so I guess more games are supporting it, but for now, a single core should last you quite a bit.
But, if you're going single core, don't get a FX-57. They're fast, but extremely expensive.
If I were you, go with a Opteron 146. You can overclock it to 2.8ghz-3.0ghz on air cooling. It's insane. According to Ownage, some guy on OCForums overclocked his Opteron 144 to 2.8ghz (?) with the stock voltage.
Nah, the opteron 275. More hyper transport links for a connection between the cpu cores abd everything else plugged into the mobo. Each of the two processor cores featre 1mb of l2 cache and it's own on-die memory controller. The newest opterons even feature support for intel's newest sse3 instructions.