Both ATI and NVIDIA (two large GPU makers) have designed low-end video cards which complicate the whole memory bandwidth issue. NVIDIA's implementation is called TurboCache. ATI's is called HyperMemory. Cards which implement TurboCache are often called "TC" models and HyperMemory is often shortened to "HM". You need to watch out for these kinds of cards because their memory systems are very different from most video cards.
Both of these kinds of video cards borrow RAM from the motherboard to use as video RAM. These video cards have a total useable amount of RAM which is the sum of both the video RAM actually on the video card plus the RAM borrowed from the motherboard. These cards are produced because it's cheaper to borrow RAM from the motherboard than it is to include "real" video RAM on the video card."