and who installs them and where do i buy?
You can buy them online from their manufacturers homepage (
http://www.ati.com), or from site like
www.newegg.com). But I just buy my video cards from Best Buy.
Like the Doctor said you can install them yourself, it's very simple, or someone at Best Buy would probably do it if you bought one there.
And personally I like the ATI Radeon 9550 256mb(currently the card I'm using) it cost me about $130 a year ago, not sure what they go for now.
Also, make sure you get the right kind of card, if you get an AGP card, make sure you have AGP slot/s.