For an inexpensive solution, pick up a PSP (Sony Playstation Portable) car charger at any local RadioShack/Target/Walmart for $9.99. I bought a pelican brand one, and it's rated for 2A (2000ma) at 5v, which is more than enough to charge your x50v while in use (even with the CF card).
It's charging plug is the exact same kind that the x50v uses, so it just plugs right into the AC Adapter dongle or cradle. I've used this Pelican PSP charger during a road trip for hours on end, with screen at full bright, maximum cpu speed, microdrive, and battery at 50%, and it charged my battery to full while watching a movie. good stuff.
As woodbane said, look for a charger with high current output (at LEAST 1500ma or 1.5A). But I would not recommend that linked boxwave charger at all. They claim that it can charge and run an x50v with it's 1Amp output, but that is not true. A couple people on these forums have shown that the x50v draws quite a bit more than 1Amp when in use + charging.
EDIT ------------
CAUTION: It seems there are two "Pelican PSP Car Charger"'s out there. Be sure to get the one that looks identical to this item at Walmart. I've heard there is another (different looking) one that does not supply the needed current.