I agree that one or the other must be made, first. Either an A500 or an A600 board. I would also say that an FPU isn't going to be of much benefit for the purposes of the card. Basically what we want (especially us A500 users) is a little bit more speed, a mass storage interface like IDE, and more RAM. Anything extra is going to make it more difficult, more expensive, and make it take longer to produce.
I recommend the first poll you make is to ascertain whether there is more demand for the A500 or the A600 version. Whatever the poll reveals, go with that one first. If it is successful, then you can do the one that came second.