The biggest reason is that most HD manufacturers only claim 320,000, 000,000 bytes which most computer-folk translate into 298GBs. 8 of the other missing 10G is wasted setting up a volume - I can only guess that the last 2G may be related to formatting and other overhead items (MBR, general ntfs overhead, root directories...).
A "500G" seagate nets me < 470G when Intel's Raid Manager first gets it. I have '2T' (4 drives) that net me < 1.4T ( it's an Intel Matrix-raid: first 500G is Raid 0; all left-overs are Raid 5 - so I don't feel _too_ bad).
keep looking for the latest Nvidia drivers - do your log files (ctrl pnl->admin tools->event viewer->system logs) give any hints about the restart issue?