dont be set on 18s. Alot of people do that. Everyone wants to go "big" and for the wow factor but think of it this way. If you ran a pair of 12s or 15s off 2 2ks and could do a 150 would it be more impressive than a single 18 doing a 146?
The trick is to maximize your space. If you only have room for 12s or 10s you need to get the biggest baddest 12s or 10s you can. You following me? 18s are cool and all but sometimes its not always optimal