Memory Issue with an XPS 600

Since I pulled out the four 512 mb ram cards and replaced them with four 1 gb ram cards, the computer seems a little slower. Since the current OS will only support 2gb of ram, am I somehow confusing it with the four sticks of 1 gb ram? Should I just use two 1 gb ram sticks or put the four 512 mb sticks back in? The only reason I keep the four sticks in is in case at some later date I go to a 64 bit OS and just so I would not lose them.