Well, it depends on your motherboard...
If you've built the 'puter yourself from scratch, you shouldn't see any problem, but if you're using a prebuilt (Dell, Compaq etc), you might experience problems...
In those cases, the motherboard doesn't have the same compability as it would otherwise, as it's supposed to only be upgraded by the manufacturer... This is why you'll see memory makes like Crucial have a wizard to let you find out what memory you can use with your (prebuilt) computer...