From guru3d: the Radeon HD 4870 series requires you to have a 500 Watt power supply unit at minimum if you use it in a high-end system, and I think that's barely on the safe side. Also recommended is 32 AMP's on the 12 volts rails for stable power distribution (in a single card configuration). Please make note of the fact that the card uses two 6-pin power connectors. The other numbers you have to check is the amps and the rails.
The HD4870 uses about 150w during normal gaming and maxes out around 200w under stress testing. A 450+ watt PSU with most of the power available to the +12v rails from a good brand like Antec, Corsair or Seasonic should handle it.
I'm not seeing very many good deals on PSUs at this particular moment. For now I would probably get this; http://www.newegg.com/Product/Product.aspx?Item=N82E168...
I agree with jyjjy. However I would spend the extra 10 bucks and get this http://www.newegg.com/Product/Product.aspx?Item=N82E168... because it will give greater head room and allow for expansion later on. Most people buy the power supply for what they need right now and do not plan for what they might need in the future. It is very important not to skimp on a powersupply because it can cause all sorts of problems down the road even damaging your system. Keep an eye out for name brand and 80 plus certification. Corsair is my favourite powersupply brand because I have not run into any problems with their products giving me nice clean energy and being far more reliable than other brands I have tried.