According to the specs: http://h10025.www1.hp.com/ewfrf/wc/document?lc=en&dlc=de&cc=de&docname=c00906129#N131, you can use one or two gb ddr2 sticks in pc4200, pc5300, or pc6400. The disclamer is for windows 32 bit operating systems, which only recognize up to 4 gb of ram (3.25 gb average with onboard video). If you try to force a ddr3 stick into one of these slots, and you will ruin the slot and possible the board. I would pull one stick out and match the ram brand. Get a 2x2 gb kit for windows 32 bit operating systems, and sell the old sticks on craigslist to help pay for the upgrade. 4 gb is plenty; ddr2 is too expensive to get 8 gb (prices are going up after the holiday sales). It is possible but not guaranteed that 6 gb will work if both pairs of slots have matching pairs of ram (2x2 gb in one pair, 2x1 gb in the other) but if the new ram doesn't work with the old ram, sell the old ram.