Memory can be tricky, particularly older RAM, sometimes it's difficult to know when will and won't be compatible.
Your best option is using the Crucial Memory Advisor:
http://www.crucial.com/usa/en/advisor
Plug in either your system (if it's a pre-build dell/hp or similar) OR your motherboard details if it's a home-built system. You'll get a list of compatible RAM which you can buy direct from crucial. You can match the specs of the RAM they list with a cheaper alternative from elsewhere and it should work too.