Quadratic speedup, IIRC - a 128-bit key can be found by brute force in (roughly) 2^128 steps by a normal computer, or 2^64 steps by a quantum computer. This applies to all brute force algorithms, so just make your keys and hashes twice as long as you think they should be, and you're good.
This might be true (I'm not that up to date on whether there are symmetrical algorithms that negate the advantage of QC), but most of the internet / world commerce relies on public key crypto.
