Question about shared system memory

My video card has 64MB dedicated memory and 271MB shared system memory, making for a total of 335MB memory, all as listed by my computer. I know that shared system memory takes that memory from the RAM, but I have 2GB of RAM so I'm not too bothered by it. What I want to know is: is the "total available memory" readout on my computer an absolute term (in other words, will I literally get the performance of 335MB of video memory, at the cost of 271MB of RAM)?

Edit: Y'know, I read the FAQ before posting this, for those simply looking at the title and turning away. There's nothing wrong with clarification. And I didn't make that link.

 

your GFX will only use the system RAM if it needs it
if you're typing a word document, you'll have 2 GB for your system + apps
if you're gaming, you'll probably need more memory for your GFX, so it'll take some of your system RAM, resulting in a lower system RAM, but also a higher "video" RAM

 

Kwak is correct, it only dips into the system memory on a need basis,disabling it wont do anything to improve system performance.