We don't spend $$$ to get something cannot be upgraded :). All the buzz lately about the X50 ROM upgrade is a proof of that.
Here is the long (but thorough) explanation:
Brighthand Learning Center: Storage. In the article, it stated this about ROM:
"Recently, a new type of Flash ROM called NAND Flash ROM (as opposed to the more common NOR Flash ROM) has begun to appear in handheld computers. NAND Flash ROM was developed to serve as a disk-like storage medium for digital cameras and PDAs. Its advantages are that it's cheaper, higher capacity, and lasts ten times longer than NOR Flash ROM. Its disadvantage is that instructions cannot be directly executed in ROM, something called execute in place, or XIP. Instead, the operating system's instructions must be read from NAND Flash ROM into main memory and executed there, similar to your desktop computer's hard disk and main memory. Therefore, a chunk of main memory is reserved for this purpose, meaning you get less memory for your programs and data.
So, how much ROM do you need? Well, you obviously need enough ROM to hold the operating system, as well as any other programs that the manufacturer wants to include in it.
Also, some PDA manufacturers have starting using portions of ROM for non-volatile storage of user data, so that if you lose power or need to hard reset your device, you won't lose your data.
Finally, is flash ROM a requirement when deciding on a new handheld computer? Not exactly. Keep in mind that flash ROM is important only if you are fairly certain that you'll be upgrading the operating system on your device at some time, otherwise it doesn't really matter. Also, just because a device has flash ROM doesn't necessarily mean that its manufacturer will provide a ROM upgrade when a new version of the operating system is released."
The article is, of course, ancient (2003). But the author continues to update the information. You can still get an idea of different storage types. Hope this helps clarify a thing or two.