The Opterons are more stringently fabricated, tested and binned as server / workstation class processors, than are the X2 desktop CPU's, and as such, will run cooler at a lower voltage than the X2's. The 2.0Ghz Opteron 170 is statistically the best selection, often achieving a 50% overclock at ~3Ghz. The 1.8Ghz 165 is the best value OC'ing at ~ 2.7Ghz, but the 170 will typically scale up highest. I had a 170 in my previous rig, (running my present Zalman 9500 cooler), and it was an excellent CPU. I ran it at 3.0 and when gaming, and 2.8 for typical use.
The 170 is a Toledo core with 1Mb cache per core, and is essentially a dual core San Diego, which is an excellent core, and probably the best 939 core AMD has produced. The 3700 that you have is an outstanding performer, and would be my only choice, as well as many other overclockers, for an AMD single core. The only X2's I would consider are the 4400 & 4800, as they are also Toledo cores with 1Mb cache per core. I highly recommend the 170 as a cost effective FX-62 equivalent. You can't go wrong with it.