It is possible to get a 2nd-user Duron for approxiamtely half the price of a 2nd-user Athlon. Maybe I was a bit misleading with my sweeping statement - but I was trying to generalise.
The performance difference is not HUGE at all. There are a lot of other dependencies, but the majority of benchmarks I have seen average out at a 10% overall difference in performance. Athlon <-> Duron != PIII <-> Celeron. The Duron 850 is a very popular chip in much the same way that the old Celeron 300A was.
I can post many links and benchmarks to back this up, but, as is observantly noted;
"If you ask 10 different techs the same question, you'll get 12 different answers."
Note: I AM a professional hardware guy who does this for a living ;-)
If you go the 2nd-user way, and get a slot A chip, you can't go too far wrong with one of the A-Bit KT7 variants. The KT7A with its 266Mhz FSB would get optimal performance from whichever chip you get.
There is no "best board", for AMD chips, however, but consider manufacturers like A-Bit, ASUS, Chaintech, Soyo and Tyan. Avoid PC Chips.
Any information I give is based on my experience, opinions and research.