I have to say that I am very upset and frustrated with adobe and Nvidia.
I Recently switched my ati 4870x2 card with a GTX470 in hopes of using the Mercury PLayback ENgine. Well I installed card, updated drivers, set the premiere settings to GPU Mercury Support, etc. After rigorous testing and long hours, GPU load of ANY activity = 0%! I have tried default effects on clips: GPU load renders at 0%. I have tried other effects: 0%. I have tried both sd and hd footage from xl2 to sony fx1 = 0%!!!
Exporting: Tried mpeg2: 0%
tried bluray h.264: 0%
GREAT GOIN adobe and nvidia.... i fell into this scam.
SPecs: windows 7 64
intel quad core 2.83ghz
8 gb ram
Is Premiere up to date with all patches? I dont believe the GTX470 was a card they initally supported but do now.
Are you working with an existing project? You may have to set old projects' properties from software to hardware rendering; this preference is saved per project file.
Back at the CS5 main page, did you go to the FAQ and read the articles?