It's a Vista trait actually.
Vista will always use a relative amount of Shared RAM that both system and video can use.
Cards with alot of memory already won't see any performance difference, but the low-end ones see quite a dramatic one as games will only use half of the on-board ram up and the system will then use the shared ram (while a game is running Dx9Ex/10) for streaming data. This frees up the other half as a "Framebuffer", which is actually quite awesome performance wise.
For example I recently bought a 6200A for my old AGP system, and one of the biggest improvements I've seen is when using multisample anti-aliasing and aniostropic filtering. As on Windows XP, Half-Life 2 uses all of the Video Ram using MSAA 2x, AF 8x at 480p.. due to this is bleeds out to the pagefile which causes some extreme performance loss. (approx. 20fps adverage)
On Vista however, Anti-Aliasing on the whole only suffers a very very minor performance hit when turned on. (approx. 45fps to 40fps) Again will the settings above (and everything on full)
The game looks absolutely gorgeous withe AA/AF at the lower resolutions and really makes it look very chrisp at the higher resolutions.
