you are going to want to match your memory exactly ( such as the timing has to be the same )
maybe you can use a program like cpu-z to see what the current memory timings are ( like in DD2-800 memory a typical speed is 5-5-5-15 ) you don't want to mix 4-4-4-14 with 5-5-5-15 ,etc.

anyway, I like to use newegg.com and read the user reviews. don't buy stuff if it sounds like it has a bad rating.

good luck with that