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.
I love the San Diego core...