bfg 8800 gts help pls

ive just bought the gts8800 but my psus only rated at 300w the recommended is 425w for a duo but ive only got a 3.4 ht so im a little confused the last time a had card that didnt get enough power it used to crash coz basically the onboard fan wasnt running.should i upgrade to a bigger psu or am i ok all my games run fine but would i get an even better fps if i upgraded my psu.test drive unlimted is a little bit stuttered but i thin thats a poor game to bench mark on.i ran future mark o6 and got about 6000 which was middle of the road.im just a little confused if i had low power would it result in the games crashing or my graphics slightly comprimised