To see what your machine supports?
I never have seen any but I have heard of pcs that have a huge amount of ram. and vista 64 says it can have like 124 gb or something like that (obviously i dont have the 10000 to shell out for such a uselessly high amount, but i would like to add some otherwise this is kind of a useless os upgrade for me.)

Chowhound
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GameSpot
Giant Bomb
TechRepublic