I have a couple of options for you that should help solve your problem.
1) Head to
www.4allmemory.com and run the simple (and safe) scan which will determine exactly what speeds of ram will be compatible with your system. (Do not purchase from this site, for it is fairly high priced generally. Shopping at tigerdirect.com or newegg.com has always worked well for me.)
2) Turn off your PC. Remove the power cord from the back and open your case. Simply look at your stick of ram and there should be a sticker on the stick itself showing the speed (PC3200/ DDR 333) or whatever it is. It will also show the stock CAS Latency and other info that is not important to finding a compatible stick.
Good luck! and I hope it helps.
PS) I noticed today that
www.tigerdirect.com has sticks of Ultra 512MB PC3200 RAM on sale today for 30 bucks after a rebate. Very good deal on what has proven to me to be a great stick of ram if you aren't overclocking.