You are good to go until your card arrives -as your chip and board have integrated graphics support.
Yes you will be able to upgrade ram how you please. The performance gain when comparing 1600Mhz and1866Mhz RAM will most likely be unnoticeable/insignificant in gaming and even most benchmarks (obviously not RAM specific ones). I would just go with the cheapest set unless there are a ton of bad review -Kingston and Corsair usually make solid products and I have had good luck using both brands personally.
You have a dual channel board so if appropriate DRAM type is used it will run in dual channel mode by default. General rule of thumb is to not mix brands/different part numbers -as functionality is not guaranteed - but I've had different brand DIMMs work on some mobos before. But more often then not you
will run into problems mixing DIMM brands/speeds on one board.
Happy gamin'!