Now that they've dropped in price, I'd recommend the Opterons myself. The 165 is available for $300, which is less than you'll be able to find the X2 3800+ for, as far as I know. It's 200MHz slower (1.8GHz vs. 2GHz), but has twice the cache (1MB L2 per core). Overclocking abilities should be pretty similar, and performance per clock should at least in theory be a little bit better than the X2's (other than the 4400+ and 4800+) due to the higher amount of cache.
I'm personally going to buy a 165 almost certainly.
