AMD Athlon 64 4000+ @ 2.6Ghz or a AMD AM2 Dual Core 4400+ @ 2.2Ghz
If you must go AM2 then the 4400+ is superior to the 4000+, you can add clock speed but you can't add another core. In many current games the 4000+ would be a little faster but in future (and those that support dual cores) the 4400+ will surpass it. You'd also notice a smoother multi-tasking experience and the weight taken off (from background tasks) would nullify the 4000+ speed advantage somewhat (in those apps that don't support dual cores).
In other apps (especially those that take advantage of dual cores) the 4400+ can be significantly faster and from what I've seen the price difference between the two is not that great, so even from a budget rig point of view buying an AM2 4000+ is a questionable move.
Edit: From those clock speeds it looks like the X2 4400+ is a Windsor core with 1MB cache per core, not a 65nm 512KB per core Brisbane, that is another thing that the 64 4000+ won't be able to "add in".