I have found that you must have a min. of 128, but just 192mb seems to dramatically improve speed over 128. I have several 500 mhz pcs(K6II and celeron) that I have put xp pro on that have 192 mb of memory and 8-10 gb hdds. Even with on-board video, they all are performing ok and very stable. Not gaming PCs, but good enough to get a non high graphics student through school with a burner.