First off, the advantage of the opterons is the bigger cache featured on every model, which only the 4000+,4400+ and 4800+ X2 chips have. Plus opterons are generally considered to be binned better.
While a 165 is slower than a 3800+ at stock, if they both overlock to the same speed, the 165 will be the better chip. So I would stay away from the 3800+,4200+ and 4600+ if your planning on overclocking, because a 4000+ at 2.5ghz will actually be faster than a 4200+ at 2.5ghz due to the bigger cache.
As far as opterons go, I bought a random stepping of Opteron 170 from newegg, and it hits 2.666ghz on stock Vcore, and Im a 100% newb overclocker. So Id say your better off aiming for an opteron, since your probably less likely to find a poor overlocking opteron than X2.