If you have a board that can do 315mhz+ and the price difference over the X2 is less than 25% (to account for the probable maximum performance increases from extra cache and overclocking headroom) then consider grabbing the 165.
However, if you want cooler, want a 10 x multiplier (meaning overclocking is easier and even a moderate overclocking board can get it's max overclock) and cheaper, then grab the 3800+.
Ultimately, with newer gen parts coming out which will be faster than the current opterons there's little point paying for any significant price permium for the Opterons as really when it comes to the CPU being outdated - by the time an overclocked 3800+ is outdated and can't keep up so will the Opteron - say a 200mhz extra overclock and extra cache is unlikely to save it. X2s and Opterons will get outdated together as a generation eg say when quad-core CPUS become mainstream and software there to take advantage of it.