You can do a C2D for 600 dollars, but its tough. For the super budget builds, like 550 and less, the AM2 Athlon X2's really cannot be beaten. The big thing with the AM2 processors, is the motherboards to go with them are downright cheap for GOOD boards. Equivilent boards are often a solid 40 dollars more. For example, the 1.9Ghz Athlon X2 3600+ running the Brisbane 65nm core (same process as the C2D) is 65 dollars. You can pair that up with a good board from 60 to 80 dollars, even with firewire, overclocking, etc - all the goodies. For the C2D, you're paying 120 minimum for an E4300, or more for an actually E6xxx series. Not to mention a descent board for it, which the minimum board I'd honestly go wtih is the Gigabyte S3 series. Even then, that board is less featured then, say, an Abit KN9S nForce 570 motherboard for the AM2. Did I mention that the 3600+ Brisbane's are hitting 2.6Ghz easily on stock voltages, some a bit more, and with a bit of a voltage bump they are hitting 3.0Ghz? That's comparable to The C2D in overclocking.
If you're interested in an AM2 build, you can look for my build that I posted in the General PC forum at the top of the index page. Or, if you express interest, I could post one up later on as well. Just trying to offer other ideas then the mindless "C2D FTW!" chants everybody pawns off.