In my opinion (albeit not highly experienced!), the postcard is really meant to be something to pique a potential client's interest. It should be eye-catching (but not obnoxious) and give them just enough information to want to check out your site or call you. It shouldn't be something where you cram as much information onto it as possible (as much as we want them to know all we can do!). You could also add on a little coupon of sorts, on the side where the address goes, or you could even add a unique code where they can get a discount on the first time they use your services. I found a website a few weeks ago where you can generate unique numbers that you could use for this, invoices, etc.