What ram configuration do you have, four 1GB sticks or two 2GB? if you have four 1GB sticks, then maybe one is faulty? Try running Memtest86+. My copy of 7 x64 has two numbers for RAM in system information, the amount installed and the amount it can use. In my case, it says Installed memory(RAM): 4.00GB (3.75 GB useable). What do you see?
2x2gb sticks, in dual channel. Windows recognizes all 4gb, but says only 3gb are usable..
Since in the official page of my motherboard (the link above) states that 4gb are supported, i thought there would be a way to make all that 4gb usable...
Have you found a solution?
I'm facing the same problem...