I checked my references and discovered the total power draw at a wall outlet (mains) for an entire pc sytem with an Intel Core 2 Quad Extreme QX9770 @ 3.852 Ghz and one Gigabyte 9800GT 512MB version used about 155 watts at idle and 227 watts at load. The Gigabyte version uses less power than the other 9800 series cards offered by other companies.
Other references indicate that a system with a moderately overclocked cpu and one video card will typically use just a little under 300 watts during a gaming session.
Seasonic has a reputation for high quality power supplies that consistently earn high marks in technical reviews. The particular model you selected has two 12 volt rails rated at 17 amps each. You should be okay if you are not going to upgrade and you do not do any extremely serious cpu and gpu overclocking.
Normally I would recommend one of the SeaSonic S12 II series of power supplies, preferably the 500 watt version. A lot of other high quality psu's are based on the S12 series.