First, I'll reply with what Vista SP1 32-bit reports to me on System A. I have 2 x 2 GB RAM installed and a 3072MB page file. Video card is a GTX 280 (1 GB VRAM) and resolution is 1650 x 1080 32-bit. Firefox and a few background apps are running - consuming 110 MB RAM under Task Manager.
Physical Memory (MB)
Total 3070
Cached 2257
Free 132
Page File 996M/6039M
Next, System B has 2 x 512 M and 2 x 1G for exactly 3 GB of installed RAM. Page file is set at exactly 1536M (or 1.5 G). GPU is a Radeon x1900xtx (512 MB VRAM), set at the same 1650 x 1080 resolution. This one runs Windows 2000 Professional SP4. Firefox and a DX7 app with background processes are open - using ~200 MB RAM.
Physical Memory (K)
Total 3112172
Available 2542800
System Cache 610740
Commit Charge (K)
Total 669380
Limit 4553604
Peak 670612
With two copies of Prime95, I'm able to load almost the entirety of remaining physical memory on either system with no persistent thrashing. (Windows always frees up a little extra space, so task manager would say ~2.98G used.)
3039.2 MB RAM displayed with 3072 MB installed, using a 512 MB video card.
3070 MB RAM displayed with 4096 MB installed, using a 1024 MB video card.
At least I fail to observe something approaching the 2.2-2.7 GB region that others are mentioning. Does anyone have something closer to the OP's system than my System B?
Thogrom's 3.5 GB theory would explain my System B if only System A reported 2.5 GB available, since it has a full 1 GB video card.
4gb is the minimum for smooth operation in Vista.
Also while 6 Gb of ram would be nice I really don't think my budget will allow at the moment.
That is the primary reason I'm on 32-bit Vista. 3 GB under 32-bit is like at least 4 GB under 64-bit. You spend more money on RAM to have the same effective space under 64-bit. If you need the space, you have no choice. I just don't need it yet.