Have you tried to run Vista 64-bit with more than 4GB RAM?

I worked with Dell support for 2 weeks to get my new XPS 720 QX6700 quad-core PC to recognize more than 3GB RAM and it would BSOD (Blue Screen of Death) each time Vista x64 started to load. Dell even sent me a new motherboard for my 2 week old PC. Then they said my U360 SCSI hard drive was the issue so I yanked it out and baselined a new SATA drive and got the same BSOD results.

Finally a Dell technician found this Microsoft Patch and it solved the problem.

 http://www.microsoft.com/downloads/details.aspx?FamilyId=91672C7C-614B-404C-850C-377541E93C18&displaylang=en

I even tested by uninstalling the patch and the BSOD returned as soon as I rebooted. I am finally able to use my SCSI hard drive with Vista x64 and 6GB RAM.

Thanks to Dell for all their help.