I've said this before, but I'll mention it again for those that don't know...
Rather than drop $100 on an extended battery, why not get one of those "Battery Extender" devices on ebay for $10? Many people don't actually need an extended battery, they just want to use their PDA for long periods on certain occasions (e.g. during a long flight). These battery extenders plug into the cradle connector on the bottom of the PDA and hold four AA batteries, like this one for the X30:
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=56154&item=5725853 105&rd=1
You can buy yourself a pack of twenty 2250 mAH NiMH AA batteries for about $25 online. Four of these batteries supplies more juice than the X50 high-capacity battery. Then you can have the equivalent of FIVE extra high-capacity batteries for long trips, at the cost of a little extra bulk and weight.
(Remember that mAH ratings don't add, i.e. four 2250 mah AA batteries does not equal a 9000 mah battery, they equal a 2250 mah battery that supplies 6V. It's really better to think in terms of watt-hours rather than mah, because mah alone does not tell the whole story).