I plan on buying a AMD ATHLON XP 2400 processor but I really don't know which motherboard would be best for this processor.

a7n8x-deluxe or usually named a7n8x-dx is best for athlon xps. Top of the line with many features. Best of all if u buy 1 u can be sure it will run a 3200xp+ with a 400fsb too if u ever upgrade. Also supports surround sound with an extremely well done integrated sound system. lastly its newest drivers are extremely stable. I got 2600xp+ on it, the computer has not crashed even once and it hasnt been turned of for 1 month now beside sleep mode once a while.

Get one with a nvidia nforce2 ultra 400 chipset. that way you have support for 400mhz fsb cpu's.

Which board is best depends on whether you plan to overclock. You can get boards for $50, $100, or $150 and the main difference is max oc. You also pay for support, quality, and extra features but if you are not going to overclock, those are probably immaterial since a $50 board will likely work just fine. But because the lower the price the higher the risk of it being DOA, the best option is to get a $50 board from a local retailer so you can easily take it back if it doesn't work. Fry's is selling the ecs k7s5a for $49 at the moment, a fast board if you get one of the 9 out of 10 that works.