I'll skip to the punchline of the article that Jim linked to: If you're looking for smooth playback of un-effected video streams, a powerful GPU won't help, since decoding is done on the CPU. If you're doing anything to the clips (effects, scaling, de-interlacing, frame rate conversions, etc.), then the GPU matters, and the CUDA/OpenCL features of Premiere Pro CS5 and later are a godsend.
But please do read that article that Jim linked to. It is (IMNSHO) the best resource for what the GPU does for Premiere Pro.
From: After Effects Mail List [mailto:AE-List@media-motion.tv] On Behalf Of Jim Curtis
Sent: 10May2012 16:11
To: After Effects Mail List
Subject: Re: [AE] 2 gfx cards in a workstation and cuda acceleration
On May 10, 2012, at 5:53 PM, Dale Baglo wrote:
I'm toying with the idea of buying a Quadro 4000 for Mac. Is that the best way to get more smooth playback of multiple high def video streams? (I don't do any 3D work yet.... but I guess if I upgraded to the newest AE I would certainly dabble in it from time to time.)