Why do I have 896MB of ''Dedicated graphics memory'', when I have a ''Total available graphics memory'' of 4095MB?? The rest 3199MB goes to ''Shared system memory''. Can I change that, so that I can get max graphics memory as I play a lot of games?
The shared memory is a ton slower. You can't upgrade the Dedicated memory as it is soldered onto the graphics card. And amount of memory doesn't matter as much as the speed (GDDR5 vs GDDR3 and the bus size)
A GTX 260 has 896 MB of graphics memory, in SLI or CF the memory does not add as each card stores the same stuff on its memory so even though you have 2 GTX 260s in SLI with each having 896 MB of graphics memory, the combination still only has an effective 896 MB of graphics memory, and you cannot do anything to increase that.
The shared system memory is just because the card is able to store some stuff on the system ram, but cannot use it as a frame buffer like it uses its dedicated memory. Once you bought a card there is no way to increase its graphics memory.
Yes, you have twice the compute power, but you don't have any more memory so for memory intensive tasks like AA and extremely high resolutions you will still run into a graphics limit at the same point if those are what were limiting you.