I was just set to purchase a Nvidia GTX 470 when i saw some statistics regarding ATI which i didn't know.
Is stream on an ATI card much better at performing the maths required for encryption and decryption?
I have no horse in this race; but as someone trying to do my own GPGPU development and who knows a number of other people doing the same, let me observe:
Getting close to peak performance on the ATI cards is harder, because not only do you have to break the computation units up into the very many compute cores, you actually have to 4-way vectorize the calculation on each of the many cores. For something simple like encryption or MD5, this may be straightforward, but it's another step you have to go through.
The development tools (a debugger and a profiler, plus an entire online development community) that come for free with the NVidia SDK are way more useful than anything that comes with the ATI stuff. It's all well and good to say that OpenCL is OpenCL, but that won't help you find a sneaky bug or find out why your occupancy is lower than you think it should be. There are commercial third-party products out there that support the ATI stuff that may just be the most amazing tools ever made for all I know, but it will cost you.