could be the psu but its more than likely graphics card drivers.
i ran a p4 1.8/xp/ cdrw /dvd drive/ 40 gig HD/512DDR/radeon 9000 128/4xfans all of a 250w psu for ages no problems.i now use a 350w because i got a new case.
some new graphics cards require more juice and need to be connected to the psu directly.most of these recommend a 300w or higher psu.
try updating the graphics drivers first
nvidia click here
ati click here
and some new games now require direct x9 to run correctly click here