If it's the "Small form factor" model, you'll need a half-hight video card and bracket. If it's the other one then a stardard card and bracket is fine.
Oh hang on. You'll need a PCIe 6 pin adapter too. I'd be very surprised if the PSU supplies one. You'll need to get a Molex to PCIe6 pin adapter... That might strain the ancient PSU in that HP, but if it's all old hardware maybe you don't care?
Lots of people (including myself) will recommend against this route as it can strain power supplies past what they were designed for. Surely though if the hardware is gathering dust... you don't care that much? My guess would be that 90% chance it'll work just fine. There is a very small (but distinct) chance that the PSU will catastrophically fail, taking other components with it... so there's my disclaimer... but as I say - if it's gathering dust anyway, do you care?