I started years ago with a PC, simply because a PC was a whole bunch cheaper than the equivalent Mac.
These days I use a dual quad 3.3 GHz Mac Pro with a Quadro 4800 driving two 24 inch monitors and an Nvidia 120 GT driving a third 22 inch. I see no issue whatsoever with graphics flicker, distortion or anything else. The major issue for me is that my 22 inch monitor is rotated, and Cubase can’t seem to remember how that works from boot to boot. I always have to reset the workspace when I start it.
On the other hand, I have no crashes or other weirdness.
I’ve also got a working copy of Windows 7 running Nuendo on my Mac Pro. I’ve noticed over time, though I don’t use it much these days) that Windows on the Mac Pro seems slightly quicker on the Mac than OS X does. I’ve never tried to measure it, it’s just an impression