Usable memory doesn't get deducted unless there's a hardware error.... If you're using 4 gb of ram with a 32 bit Os, the unused of the 4gb would be added to available video memory and not to the video card itself... It also applies to two cards in crossfire...
Here is the problem. Computers work with on or off, or 1s and 0s. This gives you two options. If you have a 32bit system, it can arrange up to 32 of them. 2^32 comes out to ~4GBs. This means the maximum amount of memory space a 32bit system can use is 4GBs.
Back in the "old days" this wasn't a problem. You had perhaps 3 slots, and memory sticks were only 128MB or perhaps 512MBs. This gave you 384MBs of ram, or 1.5GBs of ram. RAM was assigned address space starting at the beginning, while system resources were assigned address space starting at the end. None of this was a problem until we hit 1GB sticks and larger memory video cards.
Here is where things get tricky. Your OS will reserve address space based on the amount of stuff you have in your computer. The more drives, add in cards, etc that you have in there, the more it has to reserve. GPUs are tricky. Some will tell you that it will reserve 1GB for your 1GB card. From what I've seen, not true. It will reserve memory for it, but not the full one gig. There are things that your card needs to have space for that your OS doesn't care about. Applying AA for example. If it helps for you to think it will reserve 1GB then do so, but it will be a bit less.
To answer the first question, 32bit os limits your address space to 4GBs. Some of this will be taken up by your motherboard, harddrive(s), sound card with memory onboard if you have it, etc. Depending on your system, you can assume at least 512MBs. This will leave you with 3.5GBs of space. Toss in your 1GB card and your down to ~2.5GB. (probably more like 2.7 or 2.9) The rest of the address space can be used for ram.
Toss in another card, subtract another ~1GB. I don't know if CF/SLI will need address space of its own. I would imagine yes. Best to move to win7, x64.