You will need probably need a new psu and because were nice people heres a list of all the good brands and brands to avoid:
*READ FIRST* Updated Power supply guide
You will need about 500W so find one thats cheap and on that list as being reliable and you will be happy just to reiterate it doesn't have to be sli certified.
The 4830 is a better choice though as its just ahead in most tasks of the 9800gt and a similar price.