Luke, as I mentioned above, ATI/AMD cards are not the best for video editing in premiere (and possibly other editors), because the GPU is not leveraged for playback. In order to take advantage of the Mercury Playback Engine in PP, you need an Nvidia card (with Cuda).
As I also said above though, I can still edit large high def files on an older Core 2 Quad with a Radeon, as long as I set the quality to at least 1/2. On my laptop with the Nvidia card, I can go higher.
umm ati video cards handle editing Just fine.. its Premiere that does not work with Ati well. FCPX, avid and others handle Ati just fine. also your other computer specs have to be decent not just crap computer with a video card u think is good.
at work we have 3 Mac pros with Dual d700 firepro Ati cards and they will run circles around Nvidia Cards anyday. Premiere is attempting to correct their ATI problem now.