Concerning the PSU: A sturdy PSU is an important part of the system, and you want to have a little wiggle room for extra power but 875 is beyond overkill for your system. Honestly the PSU you have now is even more than enough for what you have.
As for the GPUs: Do not crossfire if you are afraid of bottlenecking. If you plan on spending money on two 7770's, I would suggest you get a 7950 instead. It is an amazing card, and fits the pricepoint you plan on spending for upgrades. If you need any suggestions, It's your preference on the company, but the Sapphire Vapor-x 7950 is what I've been seeing topping the charts, and in some cases matching 7970s.
Hope I helped!