if you use diferent speed rams , the motherboard will automactly ajust the clocks to a slower speed
best thing to do is to use rams from the same company and the same type
to activate the dual channel mode
sell the old rams and buy a new pack
ps: windows XP only supports 2.7GB ram , you have to move to vista or xp64 bits <-- and this its a little hell LOL bugs driver problems etc etc