Okay you should have 1 pci express slot according to this
http://h10025.www1.hp.com/ewfrf/wc/d...roduct=3298436
Here is a newegg link for pci express 2.0 cards $25-$50..
http://www.newegg.com/Product/Produc...25%20-%20%2450
Didn't have time to look at other merchants, but it appears the best you can get at newegg for under 50$ will be a 9500 or below.
You can get a 9600 gt for.. $74.99 though if you can spare the extra cash.
http://www.newegg.com/Product/Produc...82E16814143130