[matm347]

I'm no Opteron expert, but I've heard good things about the 165. They are also supposed to overclock better than the 4400+. Stock for stock, the 4400 would be better, as it has 400 more MHz.
The 165 vs 170 are the same except for the stock speeds, they both are supposed to overclock the same.

But you'll really want to narrow your choices to dual cores, many games support multi-thread now and show good performance boosts.

I had a 3.0 P4(2.6 overclocked) and my AMD "feels" at least twice as fast using apps, games are hard to judge due to the GPU doing so much work but with the 1900XTX you were thinking about getting, you won't have to worry about not having enough performance for quite some time.