Hi Will,
Permutations, Combinations and Probability questions can be some of the toughest questions that you see on the GMAT, although they're not all tough questions (sometimes they're rather straight-forward). As a general rule, you'll likely see 1 of each on Test Day, BUT as you do better and better in the Quant section, the Randomizer is likely to throw you more of them. As such, you could end up seeing up to 3 of each in the Quant section. Make sure that you know the basic concepts, so that you can answer the standard/straight-forward versions of these questions, but don't worry about the crazy versions of these questions (especially if subjects criss-cross, such as a permutation criss-crossing with a probability question).
GMAT assassins aren't born, they're made,
Rich