Im in this same debacle here ATM. I have yet to find the 165s under 200, so I assume the opty price cuts havent been introduced everywhere as of yet... and the 4400+ is 220 and shows no signs of dropping in price.
Id say opteron, just because they are amazing (there is like 3-5 threads all over the forums compiling STELLAR opteron OCs, however most are WC. I bet with a good air cooling you could get 2.7-2.8 without too much trouble.
Just make sure, when you DO go to either dual core, make sure your BIOS is updated to ensure compatibility, and make sure you apply the windows Dual Core hotfixes. Also one more thing to note, Dual cores dont have as much affinity to heat as the single cores do, the hottest Ive seen anyone dare to go under full load with an opty is 55C.