I'm running 1.5gigs of ram using all 4 slots. From first boot after build I had a perfect running machine. Not one issue cropped up. I do not overclock.
I used only Asus reccomended RAM.
My personal opinion is that some RAM issues are related to using non reccomended vendors or models.
You can get a list of reccomemded RAM for your MB from the ASUS website.
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Asus P4C800-E Deluxe, 3.2g Northwood, 1.5g Kingston ram, 160g Hitachi OS drive. ATI 9600XT Win XP Pro SP1