Well, that depends. About half the people here will tell you Intel, and the other half will tell you AMD.
If you want a computer that's fast now, go for Conroe. However, if you want a computer that can be easily upgraded in the future, go AM2. When AM3 (and DDR3 along with it) come out, you'll be able to upgrade one part at a time (mobo -> cpu -> memory), since AM3 mobos will be backwards compatible with DDR2 and AM2 cpus. IMO, it's a lot better to upgrade $100 at a time down the road, rather than forking out $600 or more for a mobo, cpu, and memory at the same time.