Windows users: tune out. :-)
Those of us editing with Mac Pros have historically been limited to the video card hardware we could throw at Premiere, AE, etc. nVidia has thus far been uninterested in providing other GPUs for Mac Pros, besides the older GTX285 and the hyper-expensive (and slower) Quadro-series (I have a 4K). An individual out in LA calling his business "MacVidCards" has taken nVidia to task on this decision, and made his own set of Mac-compatible GTX570s and 580s. The 570s are capable of being powered internally by the Mac without any extra power supplies or whatnot. If you want to move up to the GTX580, it requires an extra power supply so far. He's actually working on de-clocking a 580 and including cable adapters so that the Mac's motherboard can power it internally.
His work centers around performing physical surgery on the card to remove the BIOS support and add in EFI support. This is done so that when the Mac boots, the card is able to display the grey screen with the Apple logo. That may seem a bit silly at first, it actually matters. There are plenty of EFI-related functions you can get into during the machine's boot-up process to, for instance, boot off a different device. If your video card can't grok the Mac's EFI, then it won't be able to display that screen. His cards can do that.
To that end, I recently purchased an EVGA 2.5G GTX570 from NewEgg and had it shipped to him directly. In less than a day, he had the surgery performed, and the card was on its way to me. I popped the OEM ATI and the nVidia Quadro 4K out of my Mac, and replaced them both with the single GTX570. The machine booted without a problem but I was required to add nVidia-sourced drivers (vs Apple's included). No problem there... once the driver was installed, Premiere Pro instantly recognized it and started using it. In fact, the speed-up was dramatic. Previously my track videos would take a little over real-time to export to MP4 due to the scaling and cropping I do. With the GTX570 in, half real-time.
The only down side I see to this is that EVGA will in no way, shape, or form support the card. Should something break, I'll have to deal with the MacVidCards guy. Fortunately he's quite chatty via email, so if I pester him with a question he responds nearly immediately.
Anyone want a Mac-ready Quadro 4K really cheap? :-)