Newegg.com is good for 9800Pro's. Sapphire is a good brand. THis is the one I'd recommend. Just right under $200 and free shipping.
http://www.newegg.com/app/ViewProduc...102-268&depa=1
FYI, don't get the $160 or around there 9800Pro's. They are crippled versions of the card and have bad naming.
Since you have an Nvidia card right now, before installing the 9800Pro, uninstall/remove any nvidia drivers. After that, you can turn off the computer, remove the old video card and install the new one. Boot up, get the latest drivers off
www.ati.com (the cat 4.9's) and then go from there and should go smooth. And it isn't too much harder then installing memory. Pop the old one out, pop the new one in. Make sure old drivers gone, put in new drivers and your done.
And don't forget to touch metal on the case before touching the video card
One false touch of static and your done.