Keep in mind that your system can address 4ish Gb of ram, that includes your Vram (that is right, think i read it sum where?), so if you have 3.5ish Gb of system ram it can't address the entire 320 mb. Or you got a faulty card
Windows does not care how much memory your videocard has, and even if that was true, Vista 64-bit has a maximum support for up to 128GB of RAM so that isn't his problem. Please do not post if you do not know what you're talking about.
To the creator of this thread, I'd say ignore it as Vista is probably reporting it wrong.