Wrong. XP sees only 4GB total, so it can be more or less than this amount. My XP sees only 2.75GB RAM for example. Typically, GFX cards eat around 512MB by themselves, sound cards can take 64MB, the motherboard takes 1MB. Throw in other parts (network card, SLI/CF, etc) and you can actually run out of system RAM (try it yourself, its a fun way to break windows
).
In theory, if you were to CF 2x 4870x2's, you could actually prevent Windows from booting, as all 4GB RAM would be allocated to the GFX cards...(has anyone tried this yet???)