dont go to walmart. They sell trash there, literally as far as computers go.
go onto newegg.com. It's the top online computer parts retailer in the world now, and they have the absolute best customer service of any company you will ever deal with.
As far as what to get, depends on how much you want to spend,
you can check out my recommendations at
http://www.snotmonkey.com/content/bestbetsbefore buying a card though, you need to open your computer up and make sure you have an AGP slot (it will be brown and the top slot). Otherwise your choices will be limited to older PCI cards.
You will want a radeon 9500 or higher, or a geforce fx 5700 or higher, they all start around the 150 dollar pricepoint and go up from there