What type of mobo are you using. If its a Nforce2 based board then more then likely you only have 3 dimm spots. Your best option will be to just buy a 512mb stick of the same memory. More information will be helpful.
Amd Athlon 3200+, DFI NFII Ultra Infinity, 512mb Corsair Dual Channel, 9800 Pro @ 455/370
3DMark2001se <font color=red>18208</font color=red>
3DMark2003 <font color=red>6350</font color=red>