The Opterons will perform about the same as their Athlon counter-parts. The only times the Opterons are better is when you compare them to Athlon chips with only 512kb cache (all Opterons have 1MB, so that's better) and when overclocking.
The Opteron 146 is better than the Athlon 3200, even at stock speeds.
I wouldn't worry too much about AM2 myself, at least not now. I'll look into it in a year or so...
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