It's not upgradeable.
AGP graphics cards haven't been manufactured for a couple of years now. You could check EBay for a second-hand one - then you'll find you only get a tiny improvement, if any, because the CPU is far too slow for the graphics card to be able to run at full speed.
I also disagree that it was ever badass

The Radeon 9250's performance is similar to the Geforce 4 MX 440. They were both low-end cards, never intended for gaming. Same goes for the Celeron CPU. So I'm not surprised if it's next to useless five years later

It comes from the days though when PC components were built to last. It sounds like it'll probably run for a few more years at least, and it should be fine for basic use like Internet access and officey stuff for a good while yet.