Originally Posted by Thunderbird
That's quite easy actually. Your card has 256MB of dedicated video memory but it also contains what Nvidia called 'TurboCache' (in AGP times, this was just a default AGP feature; AGP aperture stuff). This TurboCache allows the videocard when it doesn't have enough memory to use system memory without asking the cpu for it.
Nvidia, ATI and others use Turbocache/Hypermemory for marketing purposes. In lots of cases the videocards show on the box then e.g. 512MB which is then for instance 256 MB video memory + 256MB system memory.
my 8600GT can handle up to 1Gb in turbocache mode but nvidia drivers tell me 512Mb