So many questions and so little time... lunch!
There is Wikipedia.
AMD = OpenCL
Nvidia = OpenCL and CUDA
CUDA is part of every Nvidia card, GeForce GTX or Quadro
Open source compiler to make CUDA and OpenCL much easier and better (OpenCL is 4-5 yrs old but a pain to code and optimize with a lot of micro management. Not today but AMD's support for just OpenCL is limiting. But something to support both technologies is a year away probably.
Dual boot is a must! Clone your system / or install Lion on 2nd drive
Setup Assistant to import from 10.6 to Lion so you have both.
Clones as backup.
Out of the box: the startup is missing on screen without the modification. The drivers are normally and only found bundled into OS X which is why you don't normally see or have to go to AMD to get the latest driver. Or downgrade to a better working driver. you have to rely on OS X and version.
That is why GTX 5xx and 10.7.3
OWC charges more than Amazon or Apple and charges for 6-pin cable. The 5770 does work - with 10.6.5 or later. It does not work with older OS - no driver. It does not allow you to boot from your 10.6.0 DVD and install.
You could order the GTX 570 (I'd skp on 560, I have looked into both but up to you). And 680s are not supported yet, and still scarce. But great card for the $499.
The GTX 570 2.5GB was the one recommended with its 2.5GB VRAM. $349.
http://www.evga.com/products/
To get a boot screen means modifying the card which means sending it to Calif and $100 + shipping.
Some people use the GTX purely for CUDA and still use ATI 5770 for their monitor.
Oh, yes, died. No card on hand as backup or spare.
Shame. I'd toss coin and then go with.... Q4K but
http://www.amazon.com/PNY-DisplayPort-Profesional-Graphics-VCQ4000MAC-PB/dp/B004 CRS78O
You could wait and see. I'd over night the Q4K for $810, then see how that does, it does have 2x the VRAM. And you could use GTX 570 or 680 down the road when you do upgrade to Lion plus.
Comparing 5770 to Q4K:
http://www.jigsaw3d.com/articles/gpu-performance-review-nvidia-quadro-4000-for-m ac
Quadro CUDA driver support
http://www.nvidia.com/object/mac-driver-archive.html