It's a bit older topic but I'd like to add a few words anyway.
I just bought this same motherboard, the ASUS P5B-VM and I also get 2808 MB of RAM reported at POST. I'm using 4 x 1 GB sticks of double-sided DDR2 533 MHz modules. Memory remapping is disabled in BIOS. G965 chipset can otherwise address 8 GB of memory (4 ranks per channel for a total of 8 ranks). I also have an Intel motherboard with G915 chipset and that one eats about 768 B for I/O device mappings and leaves 3.3 GB available even in 64-bit Windows (G915 can address max 4 GB). Why ASUS P5B-VM takes 1288 MB I have no idea. I set the integrated graphics adapter to use 128 MB and even if I counted that I would hardly get over 8XX MB. I haven't yet figured out what 1288 MB (520 MB over the usual 768 MB) could be reserved/mapped for.
BTW enabling memory mapping and using 32-bit Windows will show even less available memory, I think around 2 GB. It's probably a bad idea to be using such a combination anyway.