p4 3.2 - 200w is pushing it - 350 was the recomended about then (2 HDD 2 DVD and a power hungry p4 fan)
what you got in now? - that would be a better guide
eth is right though - you can hit the PSU with a dyson - also the case fan (s?)
but if the cpu looks like a fluffy bunny you need something a bit less staticy
I prefer one of the wife's blusher brushes (you know - the huge soft fluffy jobs) - that way tou won't dislodge anything
and a stiff 1/4" paintbrush - both bristle (not nylon) to get into the crevices
but be gentle - crack the thermal seal .... and you'll have a nice doorstop
then vac the bottom of the case - but don't get too close to the mobo