Buy the Sparkle low profile 9800GT, slap her in, and be done with it.
That's actually a nice compact system. It would make for a good portable system for people who goto lans or who want something as a media PC as well.
If you get a decent videocard, you could upgrade the CPU as well and extend the life of that system considerably as there is a lot of choice in 775 CPUs around. A quad core 775 CPU in that plus a good videocard and an SSD would get you a system that lasts many more years.
If you've had it for 3 years, make sure you clean everything out and blow out all the dust. It will help with cooling and airflow if you get a new videocard that runs hot.