PC Help: What type of RAM do i need? DDR2/DDR3?

Hello, I have a small problem with my Windows XP Service Pack 3 Home edition. I like to upgrade my Advent Desktop rather then just to buy a new PC so i decided i would like to Upgrade my RAM first but im not sure what/where to get it from? My mate said any "DDR2" should work fine but "DDR3" is the better? Could someone help me please? Thank you