How about the iMac you mentioned you didn't want with your 30" connected along-side it? Everyone I've ever seen doing professional editing has always had two displays. And possibly an additional analog NTSC display, though you'd probably want to use a switcher of some kind to output to an HDTV now-a-days.

There should be some discussions about which cards to get to make a workable Hackintosh. You should even be able to find some of the Apple BTO NVIDIA and ATI cards available for sale somewhere. ATI also has a few retail cards available for the Mac.

Using a Windows-centric card used to be problematic due to differing ROMs on the Mac boards (for technical reasons, not to thwart cross-use). I'm not sure what the situation is like right now, but I also know that ATI has released some dual-ROM cards in the past.

You might also want to take a look at what graphics cards are/were being offered by Psystar and that German guy who's currently selling commercial Hackintoshes.

Before dumping even $1000 into this route however, I'd advise you to try and get your Hack-on with whatever hardware you currently have around. Or use whatever Mac you can get your hands on for free right now. Do some testing to make sure you'll be happy with the workflow if not the overall speed. You want to be certain this is the platform you want to use before investing the larger sum.

Then you might want to spend as little as possible to get the bare minimum system up and running to get your feet wet (lower priced Hackintosh). Start making some money and then re-evaluate your options.

Aside re: Apple bare-minimums... A mini at Apple will cost you $599 or $750 with the upgraded processor option. Don't add any other hardware options through Apple as it will be cheaper to add any RAM or HD expansion yourself.