He's got an older Dell, which might mean it uses a proprietary power supply. Which means he'd have to look at one of the PC Power & Cooling Dell-compatible models. I don't know if there's any way to check besides just testing it. Plus, is a Pentium 4 going to be fast enough to really utilize a 9800GT? A 4670 or so might be a little better for that computer at this point.
I would say it's time for a new computer if you're using it primarily for gaming. But with a 460w power supply you should be able to run a new card without a problem. The 9800GT is killer but as mentioned by another, your proc will bottleneck performance.
chedrz, are you saying it's around the time to be getting a new computer? I wouldn't be too upset if you were to tell me that
That is completely up to you. If you believe you'll be able to play the games that you want to play at the visual levels you want to play them at, then there's no reason to get a new computer. With that being said, a Pentium 4-based computer is getting a bit long in the tooth at this point for some newer shooters and rpg's, but not for something like WoW or maybe a less graphically-and cpu-intensive game.