They are both currently the same price; however, the powercolor was originally $267 whereas the Sapphire was originally $199
I also have a secondary, less important question. I expect to need a new power supply. What is the minimum wattage I need to safely run my computer with this upgrade?
First of all - good choice! The HD 6870 is currently the best bang-for-your-buck graphics card.
The "original" prices don't matter. Pick the one with the better reviews. I don't know about the Powercolor model, but the Sapphire has made about 500 people very happy who've bought it on Newegg.
Lastly, the minimum wattage for a new PSU would be 400 or so. But that would have to be a great 400w PSU. Fortunately there's an amazing 500w one for sale on Newegg right now. I love mine, btw.http://www.newegg.com/Product/Product.aspx?Item=N82E16817371035