Do i need a new power supply for new video card?

i got 8600gt 256mb with a 300watt ps and i was thinking of getting a 9600gt 512mb card. do i really need to upgrade my ps?

i got another desktop with a 400watt atx ps, could i use that one if i need to upgrade?|||whts ur current PS wattage ?

I'd suggest to use a new heavy one surely, or else the performance WILL drop