In my experience the batteries swap easily from a 780-790. That goes for the AA pack as well. I don't know about the 770 though, I don't own one.
If you decide to go 8xx then all 8xx batteries swap back and forth with no problem.
One final thing, the hpc2k OS is a full 30% faster on the exact same hardware. I put a ce211 rom chip in my 790
(making it a 781
) and performance went way down. I put my hpc2k rom chip in my 168mhz 880
(making it a 890
) and performance jumped 30%
So, hold out for a hpc2k OS unit
(790 or a 780 with a hpc2k rom chip
) or an 890.
Wish I'd known you guys were having coffee. It would've been a great meet as I was already in Mississauga. Oh well, next time.