Oh no, definitely not. A 9800GT is probably ~5x as fast as a 7300GS, possibly more, depending on the resolution. Now, your framerate likely won't go up much, if any, because of your CPU, but with a little tweaking, you shouldn't have any problems getting the same FPS. The difference will be that you'll be getting that framerate with your in-game graphics settings cranked, instead of on ultra low.
The other day when I said that you would be get more of a performance gain from upgrading your CPU, that was before you said that you were using a 7300GS, which isn't alot faster than today's IGP's (onboard graphics). I still think that a 9800GT is too much card for that CPU, though. I agree with whoever said that a 9600GSO would be a much better match, and would save you a considerable amount of money, not to mention that they're considerably shorter (length, not height), and also use less power. I personally would put either an 8800GS, 9600GSO, or 4670 in that system. Good luck, no matter what you choose.