Hey,
First, as bob has indicated--its is time for a new computer. An upgrade will only help you a little--it wont give you the performance of a newer computer. That said, I'll answer your question below:
Is this your computer?
http://h10025.www1.hp.com/ewfrf/wc/document?docname=c00835809&lc=en&dlc=en&cc=us&lang=en&product=3340210
If so, it does have a PCI express slot--meaning a graphic update wouldn't be to expensive.
Before we can make a recommendation you need to let us know a couple things:
-What operating system? XP?
-How much wattage does your power supply have?--(It didnt say online--you will have to look inside the case)
If your PSU is up to it you could get a pretty decently performing ATI 4650 for $42.
http://www.newegg.com/Product/Product.aspx?Item=N82E16814161303&cm_re=4650-_-14-161-303-_-Product