The problem is, the RAM offered in the combo is different from what I was thinking (I've heard good things about corsair vengeance 2x4GB).
My question is, is it a BAD idea to take this combo and get the 1x8GB of corsair XMS ram instead of (possibly) 2x4GB of corsair vengeance? This deal saves $70 so I don't want to pass it up if it is any good!
Judging by what you linked, I'm guessing you will want to overclock. If you overclock, you will need an aftermarket cooler, and the tall spreaders on the vengeance ram will interfere with the cooler.
That said, you can get the vengeance in standard profile and still get the same performance. To answer your original question, you will be better off with 2x4 in order to take advantage of dual channel.
Talk to sales at newegg and see if they will do a swap.