AMD Radeon video cards are the best GPUs for mining Bitcoin. Unfortunately Nvidia cards don’t do very well at all when it comes to mining when compared to AMD cards.
If you already have an AMD Radeon 5800 or newer video card installed then you are in good shape to start mining coins.
I wouldn’t recommend going out and buying a new video card just to mine Bitcoins with but if you need to replace an older card you might consider buying a Radeon card instead of Nvidia Hardware..
With newer Bitcoin ASIC miners hitting the market in the near future, GPU based mining will soon be obsolete so be careful about investing heavily in GPU hardware just for mining.
If your hardware is not powerful enough to mine Bitcoins you can still earn Bitcoins using NiceHash which mines other cryptocurrencies but pays users in BTC.