What do you need the treatments to do? Privacy, sun control, decorative ?
Many arches look best left alone. Often times, when treated...you don't need to address the arch. You can simply mount a straight pole above the window frame and dress it with side panels. I have often made arched roman shades mounted inside the arch. Arched treatments can be pricey. If you ignore the arch, you can save some money.

shutter are also a good solution. they can be arched to match the window so as not to loose the design, tilted to keep the sun out or closed for privacy.