What am I missing?

I have a 500gb HD. After formating the capacity of the drive is 465 gigs. The usage chart for c:\ shows 340 gb used but the contents of c:\ is only 257.

The deficit is likely being used by such items as System Restore and/or Volume Shadow Copies ("Previous Versions").

That is most likely being taken up by the hard disc buffer. Is it a 32mb one?

Hi

The discrepency is because of how the drive manufacturers use decimal units to calculate the drive size and the computer's OS uses a binary system.
An example, take a hard drive that can store exactly 250×10/9(to the 9th power) or 250 billion bytes after formatting. Generally, operating systems calculate disk and file sizes using binary numbers, so this 250 GB drive would be reported as "232.83 GB". The result is that there is a significant discrepancy between what the consumer believes they have purchased and what their operating system says they have.
So you're not losing any space anywhere, you're just another confused consumer because the industry cannot set a standard for everyone to use. Some legislation is under way in some areas to force manufacturers to list both the GB (gigabyte, the decimal units) and also in GiB (gibabyte, the binary).