My guess would be this:
Memory LowGap: is for mapping local memory (add-on-cards) to system memory address size. It most like AGP port Aperture size or PCIE1gen 250M fixed mapping size. Extend it can solve when if "add-on cards" request a bigger size as like we would like to enable Qual SLI on NV new 295, it must be extend over 2048M. Can confirm that with 2x HD4870x2 in Crossfire I had set my LowGap to around 2176 at least. With Highpoint 3520LF PCI-E x8 256MB cache controller added into HD4870x2 Crossfire, I needed at least LowGap of 2560 otherwise windows boots to a blank window and doesn't go any further. As to how this effects windows available memory, with single HD4870x2 installed and 3x1GB memory, if I set LowGap to 2560, WinXP Pro SP3 32bit sees 1.5GB available memory. If I set LowGap to 1024, WinXP Pro SP3 32bit sees 3.0GB available memory IIRC.
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