I don't know if you're absolutely set on a roof top rack but I would advise against it. A hitch rack is easier to get to and you won't risk running your bikes into the garage door one day. I know what you're thinking, "how can you forget about a bike on top of your car" but believe me it happens all the time. I'm not saying that I've ever done it but I have this friend...
Thule and Yakima both have some very nice hitch racks that can hold up to four bike and stow nicely when they're not in use. Check them out.
Mike