Goto into Device Manager
View, Sort Resources by Connection
Highlight Memory
Press * (expand all).
Maximize the window and screenshot it. (If you know hex you'll likely see the issuse right away).
Certain devices use 'memory address space' (but usually not lowering the ceiling that far) for I/O
eg: When using Win32 with 4 GB on a Tyan K8WE you'll only see 2.75 GB because of all the devices on the PCI bus using the (normally available) memory ranges. As we get closer to 4 GB more and more people will notice this and transition to Windows XP x64 Edition or Windows Vista x64.
Many people running 4 GB, and not using the server version of the OS (which enables PAE-36 bit addressing) have noticed this for ages.
Also run WINVER.EXE, and screenshot it.
The memory ceiling on x86 (IA-32) is not actually 4 GB, 4 GB is the addressable memory range and many devices (SLI especially) use wide portions of that range for accelerated I/O purposes (if they didn't it would impair performance). Normally said ranges are not in use by memory, as most people have 3 GB or less of RAM.
Windows (Win32) actually divides the range in two (2 GB of 'range' for it, and 2 GB for applications). Don't confuse the range with total physical memory though. With more memory (say 2 - 3 GB) it tries to cram itself into a smaller range, with all the devices that have become used to there being such wide ranges available. This has adverse effects on PTE (see microsoft website) and other side effects.
It is odd the free part of the range (memory ceiling) would ever drop below 2 GB to 1.75 GB. If the free address range after 'everything happens' (Boot, BIOS configuring devices, Windows re-configuring devices, etc) is less than your physical memory this is what happens.
ie: You'll never see a PC with Win32 (excluding server kernels with PAE-36) be able to address 4 GB. Best case scenario is 3.5 GB, and often closer to 3 GB. However those days predated SLI / Crossfire solutions.
It is quite possible either a BIOS update may fix this issue (Your BIOS PnP configuration is doing something dumb), or a setting in your BIOS is flagged incorrectly for the hardware you're running. (especially if not running SLI, but some video cards are 'SLI on one card' so do bear that in mind too).
Without the above screenshots I can only speculate. (There are other threads being raised about this, and the above procedure will help diagnose it).
It isn't that it allocates memory, it allocates a 'normally unused' range, and if something is using it physical memory can not be mapped to that range (That is about as basic as I can explain it).
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