That would be an Opteron 185 or the FX-60 if you can find one. FX-60s show up in stock on the etailers occasionally for inflated prices. The Opty is in stock at Newegg now for $329. I'll spare you the preaching about what you can get from Intel for that price now, since I assume you want to keep your DDR RAM.
Ya that 185 sounds good I don't want to spend the money to replace my mobo and ram right now. I just want something to lessen the massive bottleneck I have right now with my 2 8800GTXs in SLI and my opteron 148.
I second the Opty.....of course, if your into overclocking at all, you could get a Opty 170 and a nice HS and then OC the sh*t out of it.....and probably get 2.8~3.0 no problems. (My 165 cuts out at 3.0 with a minimal voltage jump.....but my mobo strongly dislikes 334mhz bus speeds.......so I really can't tell you how fast it will go 8) )
I'm going to have to say "VETO!" on this.....
From what I've heard recently, the new 3700+'s aren't even SanDiego dies....they're actually Toledo dies with one core disabled...which means that they are less likely to overclock well, and even if you could, the Opteron 175 is LITERALLY 2 of these processors put together....resulting in better performance.