1) Relating to Rockbox (at least), the battery capacity setting only affects
the runtime estimation, so it's not very important to know.

I don't know how well calibrated the H100 battery estimation is, but the
H300 one is way out anyway, so for my stock 1300mAh battery I get a more
realistic estimation with Rockbox set to 2400mAh.

If someone can confirm that the H100 calibration is accurate, then perhaps
you could use this as a way to estimate your capacity (assuming you still
need to know for a battery swapout etc.). i.e. Measure your true runtime
from full, then try various settings in Rockbox until the full estimation is
close.

2) Have a look at
http://www.rockbox.org/twiki/bin/view/Main/HardDriveReplacement . You're
looking for an 8mm drive with a 50 pin connector.

 I was lucky enough to find a (used) Toshiba MK8007GAH on eBay, so I now
have an H380. IIRC there are at least 2 other Rockbox users (as well as me)
running this combination with no problem.

Also, it appears that Samsung have recently started producing new 50 pin
drives - the HS122JC would give you 120GB, if this is indeed the case. Get
one of those working and I'll be officially jealous ;-)

-- 
pondlife