1) Just the 6990s.
2) 6970=250W,  6950=200W (both absolute maxima though), so 2x6970+3x6950 gives you a total of 1100W only for the GPUs. I would rather play it safe and buy the bigger PSU, if you are sure about upgrading. You don't want it to die.

Make it fast, simple (both in terms of software architecture and interface), and scalable.

Calculate with exponential growth and bursty page loads. I hate it when BTC exchange sites load so slowly.

Make sure you can also process funds from Europe. Not only EUR but also CHF and the scandinavian currencies, where the big money lies

Oh, and PayPal would be great but I don't think it's feasible anymore