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.
Intel C2D E6320 / AMD Athlon X2 3800+
Gigabyte 965P DS3 / DFI nF4 Ultra-D
2GB OCZ Gold PC2-6400 / 2GB OCZ Gold PC4000
eVGA 8800GTS 320MB / eVGA 6800GS 256MB
150GB Raptor / 74GB Raptor
2x500GB / 320GB
OCZ GameXStreme 850w / OCZ StealthXStream 600w