Looking at the online manual: http://bizsupport2.austin.hp.com/bc/docs/support/SupportManual/c00059138/c00059138.pdf
It seems this computer dates from about 2003, and it has an AGP video slot. There are not many AGP cards available these days, as it is an obsolete form factor, and frankly, spending much money on one is not advisable.
2003 means you probably have a 4x/8x compatible slot on the motherboard. You can look through the meager selection of cards at Newegg here.
You might want to put in a request to buy such a card at the trading forums at Anandtech or in the Agora at Ars Techinca. You should be able to get a good used card for not a lot of money. We don't have such a buy/sell/trading forum here at MyCE.