The technology you're talking about is called Turbo Cache for Nvidia Graphics card or HyperMemory for AMD Graphics card and used for low end cards. Now how much amount of dedicated memory your Graphics card has? If it is 512 MB or higher then their is no need to allocate system memory from your Ram to it as these low end cards hardly use that much of video memory. But one thing, I don't think those cards always occupy memory from your Ram in contrast of IGP (integrated Graphics Processor or Onboard Graphics) where you need to allocate the maximum shared memory size from your BIOS. In case of TurboCache, the card only allocated memory from your Ram when it is required and its own memory is not sufficient to hold the data.
And allocating much memory to a Graphics card cannot increase performance if the Graphics card is not powerful enough. So allocating another 1 GB memory to your Graphics card won't increase any performance in demanding games like Crysis.
Read the following thread to get some clear ideas about Graphics card:
*www.thinkdigit.com/forum/graphic-c...ormation-regarding-graphic-cards-newbies.html