QUOTE
How Does it Work?
The revolutionary TurboCache technology utilizes the additional bandwidth of the PCI Express graphics bus to reach higher levels of graphics performance than traditional video memory solutions, delivering the performance and features you expect from NVIDIA graphics hardware. By allowing the graphics processing unit (GPU) to share the capacity and bandwidth of dedicated video memory and dynamically available system memory, TurboCache turbocharges performance and provides larger total graphics memory.
SOURCESeems that it'll take up some of your system memory too (your RAM that installed on your motherboard), much like onboard graphics doo.
Your card is 256 mb ram. It gets it's "512 mb card" from the cards ability to share the system memory (ram). Good news is that your card will utilize 512mb of ram (256 on the card and 256 of the system memory), bad news is that if it does take up your system memory (or even part of it) then that's less memory that the rest of your system has to utilize.