I suspect the two will perform much the same (Opteron's larger cache should make up for the lower clock speed). However, I see the Opteron is bare -- no cooling. Do you already have that? If yes, then the Opteron is the better deal.
That said, personally I would lean towards the Athlon only because I like to over-clock my systems and I can usually get a solid 15% more out of the Athlon 64 X2 (with good RAM and cooling of course). However, if your BIOS doesn't allow over-clocking then it's a moot point...
I hope that helps.