It's a trade off. You could still do Crossfire video cards @ x16 x4 and still have USB3 and SATA 6.
The performance difference isn't that great - a few percentage points at most. It's probably a good bargain on your budget.
Look here for x16 / x8 / x4 performance
And you could go with a single next gen video card upgrade instead of Crossfire 2 older cards (like adding another HD 4580 in a 12-18 months).