You may use up to 1 gb sticks in each slot for a total of 4 gb. It works with pc3200, pc2700, or pc2100 ddr ram. I wouldn't mix speeds. Check craigslist for some used corsair, kingston, crucial or samsung. You may find them for around $10/gb if you're lucky. You may use one more 512 and two 1 gb for a total of 3 gb, which is about the max windows 32 bit will recognize.