It would probably eat up your memory to put in a new one (so you add more), then you may want a better sound card (So you buy another one), and if you DO replace the vid card (buy that), need more HD space (buy another HD), want a CD-Burner (buy that) and a bigger monitor (buy that) and a better printer and maybe a scanner (buy them) and figure out how to disable the on-board systems, you'll have ended up spending more money on upgrading than if you buy a new one.. They are getting dirt cheap.
I used to work at CompUSA, and the techs HATED trying to disable the onboard stuff, it always took a lot longer than they thought.
Just tryin to save you a little $$!
Lets do some sexy math. We add you, subtract your clothes, divide your legs and multiply