WoW should run ok on single channel but just in general dual channel is better and should offer higher framerates, how much will vary by game.
How many RAM slots do you have? if you have 4 you could add a dual channel kit now and a matching dual channel kit later.
In an Ideal world where budgets don't matter the best option is just to go for 16gb of dual channel from the start, it's the safest option all round.
Buying 8 now and 8 later should work, but might very well cause issues as jojesa said.