Power Supply - make sure you get something that can handle your rig. You can get by with a 550-600W if you're just running one graphics card, but if you're going to invest in dual cards in the future, you'll want something better -and a good brand too, nothing cheap. Also, make sure you upgrade the 9800 with something a little newer. I hear the 550 is a nice one - and will handle anything you throw at it on high settings. When prices go down next year, then worry about getting a second one for SLI.
Also, make sure your case will have enough cooling/ventilation - overheating is always a problem - all that heat has to go somewhere.