Having read this paper in Tom's Hardware I began to wonder if I had made the right decision in going for 64 bit Vista with 4 GB of RAM until I found this page in Vista Clues with the interesting part given below. So if you have two 512MB graphics cards in SLI or Crossfire you could lose another 1GB of available RAM with a 32 bit system
The PCI memory addresses starting down from 4 GB are used for things like the BIOS, IO cards, networking, PCI hubs, bus bridges, PCI-Express, and video/graphics cards. The BIOS takes up about 512 KB starting from the very top address. Then each of the other items mentioned are allocated address ranges below the BIOS range. The largest block of addresses is allocated for today’s high performance graphics cards which need addresses for at least the amount of memory on the graphics card. The net result is that a high performance x86-based computer may allocate 512 MB to more than 1 GB for the PCI memory address range before any RAM (physical user memory) addresses are allocated.