Ram Conflict (PLS HELP)

I have two DDR333 256 MB DDR\DIMM Rams (Same Brands/Frequencies). I used to enjoy 512MB. And Now, when I add only one 256 DDR (No matter which one) My Computer works perfectly. But When I add both 256 Rams together, My Computer Freezes or Crashes after booting up Windows XP. I tried many Different ways but I couldn’t find a solution SO PLEASE HELP ME. THANKS IN ADVANCE 🙂