Sorry for this shameless plug, but try looking at
www.tomshardware.com
This should fill you in a little.
At current rates the Athlon DDR combo is the cheapest (and only a tiny little bit behind Intel's latest processors [Northwood core]).
the 2000+ (1.67 GHz [i think]) athlon is faster and cheaper than a 2GHz P4 (with the Willamette core).
Remember there are two types of P4 on the market, the Willamette core (1GHz to 2GHz) and the Northwood core (2GHz +)
Its worthwile noting that the the northwood core is intels latest - so the system is more likley (if they stop changing the sockets) to be upgradeable, but AMD are designing a new Athlon core to keep up so it may not be as upgradable.
For my money - Athlon DDR.
Warrend