I've created a few of those archways, the kind you simply build around an existing opening. I'm sory I don't have any DIY photos up yet on building them (I lost most of them in the great MAC hard drive crash of 2006), but I could expedite that if you're really interested. Take a look at these archways Archway-100
There is also a great company you might find interesting www.Arch-Rite.com
. I've never bought any of their products, but the stuff on their website looks pretty great.
If you need any more help, just ask.