HKEY_LOCAL_MACHINE\SYSTEM\GDI\GLYPHCACHE.
Change the value of "limit" from 8192 (default) to 16384 or even 32768. You'll want to play with this a bit depending on how much free memory you have. The more glyphs you store in cache, the more memory you'll use. You'll need a registry editor. I recommend PHM Registry Editor here:
http://www.phm.lu/Products/PocketPC/RegEdit/