The problem is, a lot of chipsets use the 3-4gb range for mapping pci DMA addressing. The i915p chipset in the 7700s uses the last 128mb of the 2-3gb range for caching (the video bios I believe), then the 3-4gb range is for DMA maps. I guess, technically, you could use 4, 2gb dimms, then have 6.9gb usable, but I'm more than certain that the 915p chipset wouldn't like that. It's like the 15-16mb memory gap that people used to have to use for some devices that were designed for systems capping out at 8mb ram. They would reserve an address in the 15-16mb area for DMA.
I may be way off, been a long ass time since I've researched this stuff.