If you're looking for reliability, then you need a Mac to run OS X. I know there are stories of everything working great on non-Apple hardware, but inevitably you will run into an issue. Trust me.
Outside of the legal issue, you can't just update without potential problems. When updates come out, they can break the system.
Simplicity and reliability...OS X on Mac.
As for the Quad-core iMac, as far as I know they don't have an express card slot. That limits using certain hardware, but probably only an issue for video editing and hardware interfaces.
They are really cool. I've set one up for a friend.
Stewart Fiori
Croma-Video Inc.
http://cromavideo.com