I've just installed 24GB RAM. BIOS sees all 24GB, and so does Windows 7 Home Premium 64bit, but as you can see in the attached grab, Win 7 is only reporting 16GB as usable. My board doesn't appear to have a memory remapping setting in BIOS, or at least I can't locate one. 'Maximum memory' is unchecked in Advanced Boot Options in Win7.
Windows 7 Home Premium 64bit
Asus P6X58D-PREMIUM with latest BIOS
Intel Core i7-930
nVidia Quadro 4000
G.Skill F3-12800CL7T-12GBTDD x 2 for 24GB