Well, I asked for a new barebones kit for last Christmas and this is what I got. AMD X2 5600+, An ASUS motherboard, 4 GB's of ram, an aftermarket cooler for the CPU, a 250 GB hard drive, and a 512 MB video card. I set it all up, and I have to admit, the 5600+ FLYS. I run BOINC on all of my computers. If you don't know what it is, It stresses the CPU by running different distributed computing projects, like Protein simulators, Climate simulators, ect. I have been running BOINC 24/7 on this computer since December 26th, 2007. And it's still going! The CPU temp idles around 40 degrees celsius and about 55 under a full load. I'm pretty sure I can pull off a lot lower temps if I install a better CPU fan, but for now it's running good!
Obviously, it isn't as fast as a quad core, but I personally think the 5600+ is great for someone on a tight budget, but is looking for a reliable dual-core.