You also need to 'scrub the disk' -- use a utility like Summit's
Hard Disk Scrubber (free).
This will make the data still left on the disk (yes, there's data there even after a low-level format and a re-install!) invisible to all but the most determined. It won't keep out the NSA, but it will prevent that schmuck who bought your machine on eBay from snagging your credit card info off some obscure temporary file.