basically shuttle PSU's are almost as good as any 480w normal PSU.
It will run fine.
and to ease your mind slightly my mate has the following running perfectly stable on a 30-odd pound 360w Thermaltake PSU (antec internals)
A64 3200 @ 2.5ghz
1gb RAM
6800GT @ Ultra
2 x 80gb 7200rpm Samsung HDD
Usual refinements