Upgrade to nVidia GeForce 9800 GTX+ - Power Supply

I've been running my computer, a Dell Dimension 9150, for about 3 years now and have decided to replace my old 6600 GT graphics card. I've been looking at the 9800 GTX+, but according to nVidia this requires a 450W power supply, while Dell cites my computer as only having 375W. I have read however, that Dell underrates its power supplies. Will I need to upgrade my power supply to use this card, and if so, would something like a 8800 GTS run with 375W?

Thanks in advance,
Tom

His psu has an efficiency of about 72%, I determined this by using a killawatt meter to see the amount of power drawn from the wall, then replacing the Dell psu with a known 82% efficient psu ( Antec EA430 ) and witnessing a 10% decrease in in that power draw.
It's said to have an output of 375 watts because that is it's max continous output rating.

Aerius, a 9800 GTX+ that uses a single power connector shouldn't have a problem running on your machine. It uses less power than an older 8800 GTS 640 OC, that I still have running on a Dell 375 after 2+ years.