the hp you have has no agp slot as it uses an asus cuw-am motherboard to upgrade you will need either a new motherboard with an agp slot & graphics card or a pci graphics card which are slower and more expensive but it is still cheaper than buying a new motherboard also pci cards are not as fast as there agp counterparts. upgrading can be achieved by adding a card yourself cards can be purchased from jungle.com at around �30-�70 here is a step by step guide to fitting a card
http://www.sharkyextreme.com/hardware/articles/vid
eo_card_install_guide/
but the simplest answer is to find a local pc shop a have a chat about what you'll need the card for and ask for a recommendation and get them to fit it.
a gforce 2mx 400 is a fast enough card for most of todays games whereas the tnt riva can struggle but would still be better than onboard. also spending �15 on a pci soundcard would take the strain off the processer and you'll hear the difference