Doing a direct comparison of the 2 GPUs is difficult, but from looking at the specs for each, performance is going to be similiar for most applications. Driver support for both is generally very good, as well.
Even though I am a strong ATI user, for your purposes, I would recommend the Quadro 4000 based on CUDA and your plans to use CS5 (CS5 will take advantage of the GPU to off-load processing from the CPU).
BTW, you can't Crossfire any Nvidia products (they SLI their cards). Yes, CUDA support can be important, especially if apps (like CS5) can actually increase processing performance (such as when rendering).
If you want to see a good site for some indirect comparisons, look at this Tom's page:
1. 16x crossfire is better than 8x crossfire, but you won't see the different in the real world. You don't even need to CF/SLI for 3D works, a single workstation card is enough.
2. For 3D works, you will get more benefit from NVIDIA cards, since they're support CUDA.
3. For workstation cards, both companies are make an equal performance of drivers, so don't worry about that.
Um, do workstation cards provide better performance than desktop video cards in the programs I mentioned?
Yes they do.. But only when you are working with huge polygons and textures which is not something which generally occurs in a single user and/or home environment.. Thus, workstation cards are best suited for industrial and studio setups whereas for home/learning purposes, a regular desktop card will suffice..
But I've read that a seperate (discreet) GPU once installed disables the on board GPU, is this true?
Yes.. Using a discreet video card requires the onboard video chipset to be disabled.. But having a onboard graphics chip is not bad as it gives you a backup display option which can be used in case your discrete card starts misbehaving..
Yes indeed a mass market GC (graphic card will suffice). And yet, take my experience. I have a core 2 duo with a mass market nividia GTX 8800 (its old yes indeed), I can still play games if I do not push to max the features with a very decent FPS (frame per second). Although while I can work on max, I cannot go beyond 2 millions polygons.
You will ask... but what th heck... why do you use 2 millions polygons?
Well I assume you work in max and other autodesk products to design games which you never mentioned therefore I can only assume as other segments of autodesk products are very particular and you would not want a quadro 4000 in first place but something better suited for other products.
Anyway so why I need a GC that goes beyond 2 millions polygons. Well, for baking of course!
I import export a lot between max and mudbox. You may be more familiar with max and Zbrush perhaps... but it still is the same function.
Just like you, I need a quadro 4000 so I can push in 7~8 millions polygons in mudbox then bake it in max which then will go back to the low polygon set up (around 6 thousands polygons).
Not only I cannot push with a GTX 8800 but on top of it 1.5 million polygons is my maxed out polygon erformanc and I cannot use more than a couple of painting layers.
My point is, yes mass market GC are suited for your needs right now and you will play well. Though professional cards will last longer in terms of performance and rendering; therefore, choose well, and your choice should depend on your budget and how you envision the next 5 years...