For windows XP, 512 mb of ram will do you good. In windows vista, 1gb will do you goo... but if you do more memory intensive stuff like playing games or running apps, get 2gb.
And for memory, try to get the fastest speed your computer can handle. For example, get PC3200 or PC4000 IF your motherboard accepts only ddr memory. Faster ram speeds are comptible with motherboards claiming to use only lower speeds.
Mix and matching of different speeds and memory amounts do not matter usually, except for certain special cases.