The most expensive ones if you plan to keep them at stock speeds. :lol:
The Dual Core Opteron 165/170 are the cheapest option you can choose from.
Opty 165 is clocked at 1.8ghz while the 170 is at 2ghz.
These two might be slower than the X2s but they overclock well, example the 165 can run at X24800+ stable with just the stock HSF, but a good 3rd party cooler is better to ensure stability. If u have good adequate cooling on ur PC, you might even reach FX60 speeds with ease. Patience is needed when o'clocking, it takes a week to ensure that ur opty 165 running at 2.6ghz stable. YOu have to set the BIOS at the best settings or sometimes flash it, run Prime Test overnight or even a day, Super PI, Memtest86.