The rack and pinion set up can be a fairly extensive modification, from you steering column on down. Special mounts need to be installed, and possibly the whole assembly will have to be changed out, including tie rod ends. I've never done this, but I would think that a faster box would be a much easier conversion. The trade off is feel though. A steering rack is a much more precise mechanism, because it is more directly connected to your suspension, thus better road feel. But is it worth it? I would only go that far if I was trying to make my car really handle, and that only after doing extensive other mods to the suspension, both front and rear.
I'm sure that there are conversion kits available that supply you with all you need.