How many watts do you think the following would use (just a random question for future reference):
Core 2 Duo E6400 - OC'd to 2.8ghz
Nvidia 9800gt 512mb
+ the usual:Â sata hard drive, sound card, tuniq tower 120 cpu fan
Just go with a 650-750 watt power supply and you should have some headroom for upgrades.
That sounds like a 300-400 watt build, even with the overclock. If you want headroom for better parts and more overclocking, get like a 650w PSU. Atm, I'd just tell you to get the 450-550w PSUs from Corsair.
http://www.newegg.com/Product/Product.aspx?Item=N82E16817139003
This is a 600w coolmax, its $38 USD at the moment, it also has a single power line for your 9800gt.
That's a 300w build right there. But dont bother buying that low, get a 500w+
yep thats 300w, get something extra for headroom - 500w or more, i think  that everyone's already said that lol