Mercury, CUDA, and what it all means

Question

[I just posted an article on the Premiere Pro team blog based on the information and questions in this forum thread.]
'Mercury Playback Engine' is a  name for a large number of performance improvements in Premiere Pro CS5.  Those improvements include the following:
- 64-bit application
- multithreaded application
- processing of some things using CUDA
Everyone who has Premiere Pro CS5 has the first two of these. Only the third one depends on having a specific graphics card.
Confusingly---because  of one of our own early videos that was just plain unclear---a lot of  people think that 'Mercury' just refers to CUDA processing. This is  wrong. To see that this was not the original intent, you need look no  further than the project settings UI strings 'Mercury Playback Engine  GPU Acceleration' and 'Mercury Playback Engine Software Only', which  would make no sense if 'Mercury' meant "hardware" (i.e., CUDA).
The official and up-to-date list of the cards that provide the CUDA processing features is here:
http://www.adobe.com/products/premiere/systemreqs/
Some of the cards on that list are only enabled if you have the recent updates.
On Mac OS, CUDA processing features of Premiere Pro CS5 require Mac OSX v10.6.3 or later.
CUDA is an Nvidia technology, so only Nvidia cards provide it.
If you don't have one of these CUDA cards, you can still use Premiere Pro CS5; you just won't get the advantages of processing with CUDA.
Here's a list of things that Premiere Pro CS5 can process with CUDA:
- some effects
- scaling
- deinterlacing
- blending modes
- color space conversions
It's worth mentioning one set of things that Premiere Pro CS5 doesn't process using CUDA: encoding and decoding.
Note  that whether a frame can be processed by CUDA depends on the size of  the frame and the amount of RAM on the graphics card (VRAM). This article gives details about that, toward the bottom.
Processing  with CUDA doesn't just mean that things are faster. In some cases, it  can actually mean that results are better, as with scaling. See this article for details.
The  term 'Mercury Playback Engine' refers to Premiere Pro. It has nothing  to do with After Effects. After Effects CS5 is a 64-bit application, and  it has been multithreaded for a long time, so those improvements are  there. But After Effects doesn't use CUDA (though a few third-party  plug-ins do).

Answer 1

Todd, others, one of the things I'm still a bit unsure about is the difference of using GPU and non-GPU Mercury when it comes to real-time playback.
When I added my cards, I guess I assumed my realtime playback would either stay the same, or in some cases improve, due to processing being conducted by the GPU.
What I have found in some of my HDV projects, especially where I have an embedded AE comp, is that I can't get anything approaching realtime playback, UNLESS I turn off GPU assistance and go back to software only.
So I'm wondering, if I can't get realtime using the GPU, do I need a better card, a better machine, or both? Are there cases where using the GPU is not advised/appropriate?
Could you clarify for me what GPU is doing or not doing for realtime playback?
W64, E8400, 8GB, GTX470
cheers
Matthew P