Athlon XP is the mainstream AMD processors out right now. Athlon MP is the dual-processors you're talking about. Athlon 64 are just incredibly powerful processors...they're 64-bit compatible instead of 32, and really expensive. In other words, the only difference between Athlon XP and Athlon 64 is the fact that Athlon 64's are more powerful and 64-bit compatible.
The best processor-for-the-price on the market right now is the Athlon XP 2500+. Buying an Athlon XP requires a Socket A motherboard. I suggest the Abit NF7-S Revision 2.0. It has excellent on-board sound and every damned feature you'll ever need for computing.
The motherboard and processor will be about $200 total.
The difference between a Radeon 9600 128 MB and 256 MB is barely anything at all. I have to suggest the 128 MB version if you really want this card. The 256 MB is pretty much useless, even on the ultra-high end videocards ($400+).
You can get a case from NewEgg.com for $20 with a 400w shitty Power Supply Unit. Notice I said shitty PSU...you'll need a good one if you're gonna plug in lots of hard drives. I suggest buying a cheap ATX case with an Enermax or Sparkle Power Supply. 400w or above with either should do the trick. If you're NOT going to have multiple hard drives, then a "shitty" PSU will do fine.
A generic cheap case from NewEgg will be $30 max. Most cases come with a shitty PSU, but if you want a good one, they'll run you $50+.
Motherboard and CPU combos are alright if they're from a reputable company. I wouldn't buy computer parts from anywhere other than
www.directron.com and
www.newegg.com .
If you're going to supply your own RAM, make sure it's at least 512 MB and PC2700 DDR or above.
If you have any other questions, fire away. I'll be happy to answer!
-Lasereth