The amount of RAM you can have is also dependent on what type of motherboard you have. Maybe you have a bad stick or the wrong type. Go to crucial.com and run their tool which tells you how much you currently have and how much your computer will accept.
How much memory you have available depends on what hardware is in your computer. A 32-bit OS's total memory addressing capability is 4GB. If you have, for example, 2 video cards in SLI with 1GB of RAM each, that amount will be deducted from the amount of RAM available to you, because the system can only address 4GB of memory in total. Other things also have to use this memory addressing space, like BIOS and network cards and such, which is why you will never get the whole 4GB amount as usable for your RAM.