I've been having the same problem I managed to get windows 7 64bit on (had to wipe the drive and do a clean install) and even the it still only recognises 3.25gigs of ram.
It is all down to the bios and the fact it needs updating and advent haven't got one, foxconn are the makers of the motherboard and the model is a G33m http://www.foxconnsupport.com/
Motherboards socket 775 G33M
Updating your bios, if you get it wrong could leave you with a unusable computer, I've just decided to get another motherboard off ebay and just swap the Quadcore chips over and go from there.
Hope this helps,let me know if you were brave enough to do the bios and if it did recognise more than 3.25 gigs of ram.