In FAQ: www.dolphin-emulator.com/faq.html#play%20in%20high%20definition

If you want to play in 720p go to "Graphics -> Direct3D" and change the Window Size to 1280x720. For 1080p simply choose 1920x1080.

I tested on Mac (except the Direct3D part), even with 1080p on a HDTV - it still looks bad! But today I tested this:

Graphics->Enhancement->2x native
Graphics->General->1280x720

The result is simply amazing. The frame rate drops about 5fps, but the result is worth it!

What games are you running and what is the spec of your Mac hardware?
For me, in the past, running Dolphin has nearly caused my Mac to catch fire.
I was pulling maybe 15fps on New Super Mario Bros. if memory serves right. (too slow to be playable).

I am primary playing Mario Kart Wii - and we got about 40-50fps for native 1x (SD) and 35-45fps for native 2x (720p). Playing 2 players dropped to about 25-35fps. Most GameCube games play at slightly faster speed.

I am using a lowest end Mac Mini (just bought a few months ago), you can check the spec in Apple Store.

Here's my answer for New Super Mario Bros. In 720p, using my Mac Mini, I got:

- frame limit off: about 50fps, but fluctuate a lot.
- frame limit auto: 60fps, with some fluctuation.

Frame limit auto is better than off - I am a little surprise. I only test the beginning of the first level, since I don't know how to play this game!