I would personally recommend repairing the old hx4705. A replacement
LCM assembly (US$155) and tools (Torx T5/T6 and Phillips #00 screwdrivers ~$10 or less) will cost less than a new iPAQ200 ($350 to $450+). Not to mention that you already have whatever software, hardware, and accessories you need for your 4705; much (if not all) of which would need to be purchased again for a different PPC model. I'm confident that a skilled physician could easily undertake this (not very difficult) repair task.
If you should choose to upgrade, you'll find that your 128MB-upgraded 4705 can command a much higher resale value if it's perfectly functional - it's well worth purchasing and installing the replacement part yourself. A visibly damaged unit (especially one with a killed screen) is worth almost nothing since it's apparent that
something happened to it, and it's difficult to assert any claims that the rest of the unit is still functional/undamaged.
The last factor I'd consider is that the 21X is still undergoing some apparent teething pains. It's increasingly obvious that HP rushed the model to market; there's plenty of complaints and problems (ranging from a buggy WM OS to trivial audio issues to serious digitizer failures) which haven't yet been corrected. I'd recommend waiting another 3-6 months (or more) before purchasing one of these units - by then HP (and xda-developers) should have submitted OS/software fixes and various PDA repair companies will have aftermarket offerings that can address the most common/serious failings of this device. Or it will become apparent that the unit is a lemon - all the more reason to be glad you didn't rush out to replace your trusty old hx4705.