You're going to need a powerful power supply. Two of those cards running at full speed can take up about 500-600 watts. Any brand will work as long as the main model is the 4870x2. If you get two different clock speeds, it still won't matter. The faster card will lower it's speed to match the slower card. Everything will still work fine. Just make sure you have all of the requirements to run the cards in quadfire.
You're going to need a powerful power supply. Two of those cards running at full speed can take up about 500-600 watts. Any brand will work as long as the main model is the 4870x2. If you get two different clock speeds, it still won't matter. The faster card will lower it's speed to match the slower card. Everything will still work fine. Just make sure you have all of the requirements to run the cards in quadfire.
Not really... AFAIK, Crossfire is a system of linking two CARDS together - not two GPU's. "Quadfire" would be having four 4870 x2's linked together... It's like a motherboard - If a motherboard can support dual core processors, it doesn't mean it can support two actual processors.