your video card is NOT sharing the system memory. its using a chunck of the address space. 32 bit is limited to 2^32 physical addresses which it has to use to access all the hardware. you move to 64bit you gave enough addresses for the GPU, other stuff, and all 4GB of ram. you're supposed to be able to enable PAE in 7 to get virtual addresses and be able to use more than 4GB. I've never had any reason to mess with it though. http://en.wikipedia.org/wiki/3_GB_barrier
That clears a lot of things up. Thank you. Just to confirm whether I have understood it completely, I want to ask a followup question. The 'Advanced Settings' tab in the 'Display settings' shows:
Total Available Graphics Memory: 1295MB
Dedicated Video Memory: 64MB
Shared System Memory: 1231MB
1. So this means that my video card is using up 1231MB worth of address space and leaving the rest of it for the operating system, right? And if I install 64-bit Win7, the 1231MB worth of addresses required for the video card will be available outside the actual RAM due to the availability of 2^64 addresses and the operating system will be able to use the complete 4GB RAM for its own purposes.
2. Also, since the video card takes up 1231MB worth of address space, does this mean that memory-mapped IO is being used?