If you need to create a brick arch that will be noticeable....
If you don't have Photoshop or other such program to create an arch texture....
If your @%@! retentive....
Here's a tutorial for creating an accruate brick arch with the joints radiating from the center. It's fairly easy, but time consuming, tedious perhaps. This is really only good for close-up shots. If the brick coursing of the arch won't be noticeable in your presentation, I wouldn't bother.
I've deleted the tutorial from this thread. Please see my other post - Brick Arch (revised) - for a revised tutorial on creating an arch from scratch. I will add a second tutorial on adding materials to an existing arch soon. Sorry for any confusion.
Revised Tutorial