Actually 32 bit systems support, in theory, exaclty 4GB of RAM, however most systems will hog out aroud 500MB to 750MB for "housekeeping". 32 bits is 2 to the power of 32, or 4,294,967,295, which is exactly 4GB.
Also remember that PCI-E maps video memory to this available address area. The larger your video card memory, the less you might seem to have, although its all being used.
The big, expensive, answer is to go to a 64-bit system. basically 64-bit allows more RAM than you could ever need (16.8 million terabytes) although most manufacturers place an artificial limit in there to clamp it at, say, 128GB (Still a bloody lot)