Graphics card

More information would really be useful.

- What are you planning to use it for? Gaming?

- What is the native resolution of your monitor?

- How powerful is your PSU?

Without that, no one is really going to be able to help you at all.

Might be easiest to just tell us what model of HP or Compaq computer you have.

Hewlett Packard Compaq d530 CMT (PB600A) PC Desktop

http://www.dealtime.com/xPF-Hewlett-Packard-D530-CMT-P4...

same computer, and i need it for autocad and maybe some games idk

Ouch... ok. Only got PCI or AGP- no PCI-e. That makes things much more difficult as a decent AGP card is really expensive and under-powered compared to PCI-e cards. PCI cards are really only slightly better than integrated graphics. How much are you wanting to spend?

not too much, the cheapest decent card that will do what i need

http://www.newegg.com/Product/ProductList.aspx?Submit=P...
That is what Newegg has that will possibly fit in that machine, and here is Tom's GPU heirarchy.
http://www.tomshardware.com/reviews/best-graphics-card,...
Basically, you'll want to find the best for the cheapest from those lists.
This 7300GT might be your best bet, even though its open box.
http://www.newegg.com/Product/Product.aspx?Item=N82E168...

would a GeForce 7900gs work with my computer?

which one?

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=3703...

It should be fine. It doesn't use too much power, so I wouldn't be worried.