Can OpenCL be implemented on systems which do not support CUDA? I have a relatively inexpensive HP bought about 3 years ago that, well, just won't run CUDA even with an nvidia card :-)
If you're still talking about about NVIDIA cards, then no; their OpenCL implementation is based upon the CUDA architecture (technically, CUDA is the device architecture that supports GPGPU programming, not the language itself). That said, videocards are cheap these days, as long as you don't need the high-end: you can snag an entry-level Fermi card (GTS450) for right around $100 these days, or less if you go with something GT200-based.
I don't think so, better check on NVIDIA forums to be sure ...