On intel hardware this depends on:
. the amount of main memory in the system
. the OS
. the chipset
For example with Windows XP some main memory was statically set aside for video memory. And another half was dynamically added if demanded by the graphics drivers up to a limit with could be set in the BIOS.
The intel GM965 (X3100 graphics) can use up to 384 MB, while the GM45 (X4500 graphics) can use >512 MB. XP respected the DVMT settings of the BIOS and wouldn't allocate more than that. With Vista the video memory allocation is entirely dynamic and thus smarter.
Section 3.6 "Graphic Memory Allocation" of this document covers some more aspects of this:
If you search the intel site for "DVMT" you'll find more, but almost all of this applies only to older chipsets and only to XP. The above document is the more recent resource.
The 2.8.x intel video drivers for the GM965 (X3100) statically allocate 256MB, which is less than XP/Vista can use on this chipset. I'm not sure how much it sets aside for the X4500 archs. However it doesn't seem to be dynamic at all.
This document of Microsoft describes the differences in memory reporting and management of XP/Vista on discrete nvidia and shared memory intel GPUs: