A couple of checks before you get your video card:
1. Open up your PC case and note the specifications of your Power Supply Unit. Total Power Output and Amp. rating on the +12V rail. Match this with the requirements of your card. Some cards also require an external PCI-E power connector.
(If you intend to use the 9800GT for a long time, if I would suggest switching to a reliable 400W+ PSU)
2. Determine the dimensions of the video card you intend to purchase and check to see if it will fit well inside your case. There shouldn't be any hardware component in its way. Also note the power requirements of the card.
3. Last but not the least, make sure you have a good enough computer for the card you choose to buy, to avoid a bottleneck. A good CPU and enough system memory (RAM) is always ideal. If your requirements from the card aren't too demanding, you could just settle with a HD5670.
Good luck!