What your computer components require will determine how powerful of a power supply you need. There is more to picking out a power supply than just finding one with the highest watts in your price range. You also have to consider the amps on the +12v rails. Note that SOME psu spec labels merge or keep the +12v rails seperate or list incorect amp ratings---all of these are mostly seen on the "cheap" power supplys and can be taken at face value on quality psu's.
I reccomend you take a look at some
www.jonnyguru.com reviews. They are very indepth, covering the TRUE values of the specs label. Some have amp drops at high loads for instance. High load only happens when your power usage is nearing the maximum rated wattage of the psu.
Specifically, read
<THIS REVIEW> as it shows you what cheap power supplies are, how they differ from the specs label, and what happens when they're pushed hard.
Now if you were dead set on getting the 9800 GT, and not going to overclock or have a bunch of other gadgets (raid, plethora of usb devices, case lights) I would reccomend an Antec Earthwatts 500w, as it does exactly what the specs label says it can AND they are very inexpensive (purchased mine for $35 shipped).
And for the record, use one +12v rail line for the video card and the other for everything else and try to avoid using power plug adpators to connect things when possible.