Graphics cards are pretty difficult. They probably last the shortest time period of any part in your computer as far as gaming upgrades go. It ultimately depends on the resolutions you play at, and how you set graphics options. If you want everything to run at Very High levels, with Anti-Aliasing turned on, then the 9800GT is probably at the point now where new game releases will start putting pressure on it.
I use an 8800GTS (G92 512MB version) and play Crysis at 1440x900 @ Very High and get like 26-29FPS in the beginning jungle levels. My card technically should be slightly faster than the 9800GT I think. Granted Crysis isn't the best game for balancing a video card's true performance. Other games like CoD4 would play beautifully with a 9800GT.
The reality is this. Spend what you can afford on a video card. Get the best you can afford today, and just accept the fact that it'll be more challenged by the games released tomorrow. It's just how the gaming world goes. Nothing wrong with buying a 9800GT right now, as it plays stuff fine. You just may not have every single setting maxed out, with max resolution, and 16x AA.
For $100, the 9800GT is the best bang for your buck. Again, for $100.
If you can afford an HD4850 (and find a really good deal on it) it's probably the best overall bang for anyone's buck.