next time before you buy ram , make sure you know what kind of ram you are using right now especially if youre going dual channel with the old ram + new ram.
anyways go to www.crucial.com
and do a system scan ( select from the left hand side panel ) and then tell us what speed your old ram is running at....PC3200....PC2100...PC2700.... or you can also tell us if its DDR400...DDR333....like that and we will see if its ok to mix with your new ram or not. sometimes switching the slots of where you place the ram can cause problems too
for more info, check out the RAM guide on this forum