Intel's Extreme Graphics, Extreme Graphics 2, and GMA 900/950 parts all dynamically adjust the amount of memory they share from the system, taking only as much as they need at any given time. But when you're gaming, they're going to be taking all they can -- the Extreme Graphics parts will steal 64MB, and the GMA parts will steal as much as 128MB, so you can subtract that from your total amount of memory to determine how much memory is left over for the actual game you're playing.
ATI's Radeon Xpress 200M and its older integrated graphics parts (Radeon 7500/9000/9100 IGP, 320M/340M IGP) use a set amount of system memory and this does not dynamically change. It must be set in the BIOS of your notebook (see your manufacturer's site for details on how to do this). Because of this, while an Intel part which will draw as little as 8MB of system memory when not in use, the ATI part will always be drawing the same amount (usually 64MB or 128MB) from your system memory, and this can severely cripple performance.