If you have a 32-bit OS, you can only address 4GB of memory space. Typically, aound 256MB is reserved space for the OS/mobo, etc. The rest is typically taken by the GFX card.
So, assuiming your GFX setup is taking 1GB (a 4870, or a CF/SLI solution), the maximum amount of system RAM you will have for use is 2.75GB.
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