lol i would get the opteron 165.
if you're building your own system, you may want to overclock it.. in which case the opteron's are very good.
2.4ghz is easily reached with that proc.. with stock cooling.
basically you get opteron stability for cheaper than the 170.
but yes, i definitely agree with praetor... if you had to choose the opty 170 or X2 3800+ get the cheaper of the two.
cache makes a slight difference, but the opteron's have a slightly relaxed memory controller.
basically, the opty will perform slightly better... maybe 1-5 fps in a game lol
but either will easily meet system requirements for any game out there so you'll be fine...
plus they're both dual core which cuts time in almost half for most converting operations.
my old 3400+ (2.2ghz 1mb L2) converted a divx to dvd in about 55-60 mins, my new proc does the same operation at 2.6ghz in around 25 mins...
and still only uses about 88% of my proc tops lol
definitely worth getting dual core whether you use all of the cpu resources or not.