|There you go - HyperMemory. A HM card has a fixed amount of memory & "borrows" the rest from the system as it needs it. Some HM cards have 64MB fixed, some have 128MB, some have 256MB. I believe an HM card can "steal" an equal amount of memory as is fixed on the card. nVidia uses a similar technology called TurboCache.|
The way they are marketed is somewhat deceiving though because they include the borrowed memory in the product description. Your X300 SE probably has 256MB fixed + 256MB borrowed & is marketed as a 512MB card.
Here's a review of a 256MB X300 SE HyperMemory card that explains how it works: