How important iz a psu any way.?

i am upgradin ma sys to E6600 with an asus boardd probably with a inno3d geforce 6600 512 128 bit card and 2 to 4 gbs of ram..and 80 to 120 gb hd or may be 2 hds 40 gb + 80 gb.... i dun know what to do abt the power supply.. i have no clue abt it i dun even know wht to buy for such a system ..... and i am no sure if i shoul use the psu of ma old computer almost 3 to 4 years old now.... ?? WHAT TO DO??? what should be ma requirments and plz reccomend ma a good cheaply priced pus if needed.