1) Control Panel - Administrative Tools - Computer Management - Disk Management and look at the picture at the bottom.
2) The vendor describes drives in "gigabtypes" (GB), or 10**9 bytes. The system and tools like GParted describe drives in "Gigabytes" (GiB), or 2**30 bytes. 320 GB is the unformatted capacity, or 298.02 GiB. You haven't lost anything.
Note that "GiB" is a fairly recent term and you will not see it in the system tools,which will say GB. The difference of the first letter being capitalized is even more subtle, and not often respected.