hedwards: 32bit versions can't reliably handle more than 3GB of RAM. At this point anybody using more than 3GB of RAM would do well to use a 64bit OS. For historical reasons the OS uses portions of the address space for things other than RAM.
Typically that 4GB of RAM will become 3.25GB or something of that nature. Not the worst thing in the world, but it is a waste of money.
Ralackk: It is 4GB of ram or there abouts for a 32bit system. The reason you may see less is because it includes intergrated graphics chip ram and your graphics card.
Nothing unreliable about it; it's just wasted space.
The problem is, it's hard to configure just 3GB RAM when RAM wants to be installed in pairs. Either you get a motherboard with 4 RAM slots and install 2x1GB and 2x512MB, or you install one 2GB and one 1GB stick. The first way is expensive, and both ways are slow. Slower than installing 2x2GB and letting the unused address space go to waste.
They're slow because in the first case, you can't get DDR2-800 or DDR3 in 512MB sticks, so you have to fall all the way back to DDR2-667; and in the second case, you lose interleaving, cutting your memory bandwidth in half.