The hardware MPE will always be significantly faster than the software MPE. When you use an ATI card then you are using the software engine. That is basically all CPU threading for the entire process. Any Nvidia card added to the supported list will take allot of the processing off the CPU such as effects and scaling. That leaves the CPU to handle the encoding ie why you should have an Nvidia card. Any of the 400 or 500 series cards with 1GB of video ram will work with the MPE if you add them to the supported list. The 560 and 570 cards are the ideal ones to get if you can afford them. So yes you need an Nvidia card.
I completely agree with Eric on this. The fastest HD 5870 graphics card takes 88 seconds on the PPBM5 Benchmark
The best score on the same MPE test in that benchmark, with a GTX 480, takes 3 seconds, almost 30 times faster. On the MPEG2-DVD test the HD 5870 scores 191 seconds where a comparable system with the GTX 480 scores 25 seconds (due to the hardware assisted scaling).
You will see huge benefits from changing from ATI to nVidia in video editing with PP. Maybe not in games, but we are interested in editing performance here.
Oh wow. Thanks for the info.
I've decided that I'm just going to sell my 5870 on the cheap and buy a 560 ti. A bit more than the 460 but apparently well worth it!
Given the marginally higher core count, you're not likely to see much difference between the newer 560 over the older 460 for that extra $100.