2 sticks lets you run your RAM in dual channel mode. This allows for imporved performance, so it's better.
Consider other brands besides Corsair, as they tend to be a tad over priced. OCZ, Mushkin, TeamGroup, Centon, are all good brands.
An Athlon XP 3200+ will not work with an Asus A8n-SLi motherboard. I assume you meant an AMD A64 3200+, otherwise you are pairing a s462 CPU with a s939 motherboard, and that's not compatible.