Get the Ubuntu Install disk and run the memory test.. the memtest takes a long while, but you will know for sure if there is a HW issue with the RAM.

Another possibility is whether the disk is a win7 pro 64 bit but SP1? I'd trust SP1 right now over non-SP1 for better support for Sandy Bridge. Regardless, even with SP1, especially the newer Intel chipsets, you may need a boot-driver for AHCI. I did have to manually load drives on at least one occasion with Win7 SP1..

Other than that, yes, 64bit Windows support a crap load of RAM