In terms of cost/performance ratio, the Duron on either an Asus or Abit board are hard to beat. Depending on how serious your gaming fetish is, you could look at a Tbird. But since most Duron 600's overclock to the 900 range they are hard to beat in the "bang for the buck" department.
If you aren't going to overclock, then AMD is significantly cheaper at any speed grade.
Example: Abit KT7 $147.00 shipped from microbarn.com
Duron 600 $62 - 70 shipped, various sources
Total = @ $210
The other thing to keep in mind if you decide to go AMD is that another price drop is rumored for October when the next speed grades are introduced.
A few reviews that might be interesting:
Abit KT7,
Another KT7 review
Comparison of Duron 700 vs Tbird 700
Followup, part 2 of review above
Duron vs Celeron
Good luck.