Athlon XP 3200+ uses a 400MHz FSB so you have a bit of a bottleneck going on.
You can buy any 184 pin DDR-SD RAM and pop it in, but regardless of how fast it is the memory will only operate at speeds equal to the slowest operating stick. In laymens terms, if you buy PC3200 (which you should since it's the proper speed you need anyways), it'll only operate at 166MHz on par with your PC2700 so it's sort of a waste.
Also, do not buy RAM from Best Buy, and scrap whatever RAM you have now. Sounds like nothing more than generic crap and I'm certain it'll register errors in memtest before long. Go to newegg.com and buy yourself some nice value DDR400.