As the old motherboard doesn't like Bartons (the XP2800+) because of bus speed, you don't have much choice.

I hope you have at least DDR-2100 RAM already, that can be placed on the new board, which supports the new 400MHz DDR too

if you already have a sound blaster audigy i would use that. And PC2100 to 3200 can be used with the AMD Athlons, anything above 3200 i think can be used but it wont increase preformance because of the Processor.

Dual channel gives an increase in performance. Two of the exact same RAM put in the first and third slot of the A7N8X Deluxe will give you a bit more power. so if your willing, pay another 90 dollars for another 512MB PC3200. I wouldnt recommend gettings two 256MB chips cause if you plan to upgrade u are pretty much wasting money. If you do not want to pay an extra 90 now just get another 512MB chip that is exactly the same. Same brand, same type, same everything.

Hope i didnt confuse you, i kinda confused myself