I find that 2,200 mAh batteries are fine. As I live in the UK I bought mine from
7dayshop.com for £7.95 (about US$15), like
PsionAndy a d
Alba Joe. I was so pleased with it that I bought a second one.
I got a Sena extended battery case for it and with this case you can sync and charge the battery in the cradle without removing the PDA from the case. Very convenient!
I think that if you go for a larger battery than the 2,200 it increases the thicknes of the Axim so much that it will not fit in the cradle at all.