No, the limitation of 'accessible'/addressable RAM in an OS is down to the OS.
Your are using Vista x64, which is 64-bit aware, and as such can address 2^64 individual locations, which is somewhere up in the range of...a lot, I just haven't a calculator handy and can't remember the value!
Given this, every location that needs an address, be it RAM/GPU/BIOS/IO will get one without issue.
You won't have any issues using that card with any amount of RAM your machine can support!
Hopefully that helps (and I've got it right!)
