I don't know what kind of quality of business you will get from the Yellow Pages...
From past experiences in OTHER fields, most of the people reffered from the Yellow Pages were what I would call "pain in the *** customers". For example. had a good friend that ran a painting business (doing mostly commercial stuff). He would get tons of phone calls from people asking for them to come paint their kitchen...After awhile they didnt' even want to bother with them since they were doing so much bigger stuff (ie a whole builindg etc)...
So think of this....If you put an ad in the yellow pages, will might get a lot of people wanting pesky wash & wax's and only want to pay $25 bux to do it.....In other words, the people who want a lot for a little. Where as with refferals you can somewhat control what kind of business you will be getting (to a limited extent). Do you do mostly "higher end" stuff? Or do you just try to put as many through as you can?
Either way, it sounds like you're growing to the point where you will need a helper regardless of whether you place that ad or not. Remember, the more your keep doing this, the bigger your word of mouth is going to get. Those are your most loyal, and frequent customers. Perhaps stick with that and find another media communication in which you can properly target the customer you're looking for?