I have a HP Compaq, windows XP ATX motherboard. and it's real slow. so I found out it was the Power supply I got a new one but have not installed it . the computer is dead, it wont start. but while I was looking arounf I found some 512 mb memory I had and there was an empty slot. I put it in . but the one that's in there is big memory like ???900mb ram while the one mI put in next to it is only 512 mb.. SO Will it work as a combo? or do youhave to have 2 900 mb?