Horses for courses mate, it all depends what you want it to do. If you're just after a card to do some OpenGL work on then a cheap one would be fine, if you're wanting to be able to play the latest games like Half-Life 2 and Doom 3 then be prepared to spend a lot of cash.

A good place to start looking is Tom's Hardware, they have a lot of reviews of kit and give some insights on what you should be looking for.