I can speak directly to this. I have an i7 2600k on a main machine AND a machine dedicated to FX processing through nebula server. I used it for a while and then recently bought a GeForce 570 GTX for the main machine. The CUDA processing is absolutely flawless on all reverbs which I use a few plates, rooms and halls for each mix. it was well worth the purchase. If CUDA can process normal nebula some day, that would be great... but until then it REALLY lets my processors breathe while allowing me the amazing reverbs and plates which is one of Nebula's strongest abilities.
One question... I'm using Nebula in Cubase 5, and to work around the problem of audio spikes when initializing a new Nebula instance, I have followed the recommendation of using the Nebula Reverb (threaded) plugin instead of the regular version.
If I install the Cuda framework on my computer, will all my Nebula instances use Cuda instead of the CPU?
Seems to me like they would. If they do, how many instances can I expect to use at one time with a GTX580?
a 580 is a MONSTER. You would easily be able to run 5 or 6 reverbs without a single hitch on the CUDA. I have noticed after CUDA is working at 30%, there might be glitches but I haven't confirmed any reproducable cause so don't quote me.
Now to fully answer your question... you are going to have to get a little messy to accomplish this... you need to create a COPY of the nebula reverb plugin DLL and rename it to Nebula3 CUDA.dll per the instructions in the FAQ here on the website and a difficult to locate forum entry that obtains the exact process for getting CUDA to work... The reason is that you can't let CUDA process everything because it simply can't do multiple Kernels at this point. so it's but here is a problem you WILL HAVE...
to edit the hex code inside the dll file manually so cubase will let you have two plugins with the same 4 digit vst code. Otherwise you won't be able to use CUDA and normal reverb side by side. Certainly not for the faint of heart and can seem fairly involved if you aren't totally familiar with hex editors.
You will need VSTSPY32 (find the nebula code) and XVI32 to edit the code in the copied DLL so that you can have a nebula reverb and a nebula CUDA recognized at the same time by cubase.
Good luck. I wish this was easier but hopefully you will be able to get somewhere with the info thus far.