Buying a larger power supply won't overpower your system. Your components will only draw what juice they need from the power supply. I usually advise going a bit above the minimum wattage required, as there may be some factors in your system not considered in the recommendation (i.e. additional hard drives or other stuff that's drawing power). 450 ought to do the trick for your system.
For the future reference of those buying new PSU's, wattage isn't all you need to be looking at. You should make sure you have enough amperage on the different rails of the power supply (particularly +12V) to power more demanding components. In your case Billy, that card oughtn't give you any trouble. Good luck!
Final edit, I swear:
Minimum of a 350 Watt power supply.
(Minimum recommended power supply with +12 Volt current rating of 18 Amp Amps.)
^-- Just pasted that from Newegg - for the EVGA GeForce 9500GT 1GB model. That's what I was talking about.
This ended up being a tad longer than I intended. Sorry. LMAO