Others have also reported little or no performance improvement with the superior card. You're not alone there.
What are you doing that you expect to be accelerated?
The CUDA acceleration features only accelerate some things: some effects, scaling, deinterlacing, and blending modes.
Here is what I think is going on. I made and rendered some logos and backrounds in Lightwave and After Effects and had to size them in PP. I think like you said, it just isn't going to give me everything realtime. I notice however that when I just drop a couple of vid's and simple title on timeline and add color correction it plays well. It's just those graphics and sizing them. You never know how involved other peoples projects are that are talking about great results. They could just be using two vid layers and a title. So it's all subjective.
One thing, if you can answer? Why don't I get a red line telling me it needs to be rendered, even when I stack a bunch of video and it doesn't play without rendering. I'll search the forums for an answer. Thanks
The preview bar colors are just rough suggestions, they make no promises, here is what they mean:
-Media matching the sequence with a simple codec has no color bar. This should play realtime.
-Media with an expensive codec, with accelerated effects, or mismatching the sequence has a yellow bar. This is likely to play realtime.
-Media with unaccelerated effects or other non-trivial software processing has a red bar. This may not play realtime.
-Rendered previews have a green bar.
The command to render effects in the work area will render red segments, the command to render entire work area will render red & yellow segments. Other then to check for the presence of a CUDA card supporting accelerated effects, we do not guess at how well a particular timeline will actually play on a particular system by looking at anything like processor or disk speeds because any such check is inheritly problematic. This system gives you the flexibility to see if your timeline will play without preview files, and if needed force preview renders for anything red or yellow that will need it.
If we did not yellow bar segments with CUDA effects then it would be impossible to render them even in an extremely complicated timeline. If you have a silly looking segment with 10 CUDA Gaussian blurs on HD footage it probably will not play in real time so there still needs to be a way to force a render. Likewise we yellow bar AVCHD to give users of weak computers the option to render it. At the same time red bar does not have to mean rendering is absolutely required to play in realtime, on sufficiently powerful machines many software effects will still play back in realtime even on HD footage.
Like Todd said, only some effects/transitions/scaling/blending/deinterlacing are accelerated. Your stills have nothing to accelerate. I suggest you run the PPBM5 Benchmark and see what a difference a day makes, sorry I meant MPE on or off, when you have a complicated timeline that can profit from MPE.