The ASUS motheboards are fussy with the RAM timings and many cheap OEM memory modules fail on these motherboards because of this factor. For Asus motherboards I generally use a good quality memory, my prefered brand is Corsair as they have a lifetime warentee.

http://www.ebuyer.com/UK/product/54473

With regards to AMD AthlonXP you are better off buying a AthlonXP mobile if possible because although these are still socket A processors they produce less heat and run at lower voltages than the desktop models but will fit in the same motherboards... the mobiles have better stability and tempratures... they also overclock better if thats your thing.

A good cheap graphic card to run with a SNES emulator would be a Nvidia Geforce 3 Ti500 , the golden samples are the best if you can get hold of one.

Mame is the best emulator if you want that kind of thing as it emulates classic arcade games

http://www.mame.net/
http://www.mameworld.net/