The results between the X2 3800+ and the Opteron 165 should be fairly similar as while the Opteron is 200MHz slower it has double the cache. AMD markets the doubled cache as giving an additional 200MHz although in reality it only really makes a difference in certain applications.
In general, the A64 4000+ should be faster in games because of the large 600MHz difference in clock speed. In regular applications, the Opteron 165 will be faster because of its dual core nature.
Those links were to Tom's Interactive CPU Charts so you can flip around to compare in games and applications that interest you.
Good luck! I can not seem to get my Opteron 175 over 2.7GHz.
Using 4 sticks of Corsair (2-2-2-5-1T) even at 2.5-3-3-8-2T
Oh well, it runs stable at 2640Mhz tho...
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