Currently Being ModeratedFeb 8, 2013 9:36 AM (in response to m_r_nadh)
There is always going to be a discrepancy between Disk Utility and the Finder, because Disk Utility is giving you a literal number while the Finder is omitting certain things that might as well be free space. For example, with Time Machine active, your machine stores local snapshots. These backup files, however, are thinned automatically (or even deleted altogether) by the Finder if free space gets tight enough. So, the Finder does not report that space as being in use.
Edit: As for your performance problems, see my Mac Performance Guide.