here is the guide for that machine:
http://bizsupport1.austin.hp.com/bc/docs/support/SupportManual/c00765936/c00765936.pdfYou have a standard PCI-E x16 adaptor on the motherboard (page 16) so ang PCI-E graphics card will work. there is a large range of these so may need a few more details.
What will you be using the machine for? gaming or spreadsheets etc.
What sort of budget are you on? some are very expensive
thanks