Which are the differences between the various PalmOS devices ?
Basically, all PalmOS-based devices share a Motorola DragonBall™ CPU (a member of the famous 68000 family), some RAM, some ROM (or flash RAM), a 160x160 LCD touch-screen display, and an RS-232 serial port.
Besides that, there are many differences between the many devices of the PalmOS family; first of all, there are many different companies that build PalmOS devices, and more will come; each company provides its products with peculiar features, both on the software and hardware side.
As for now, please have a look at this table:

Device	CPU	RAM	ROM	Display	Expansion ports
Pilot 1000	DragonBall 16.6MHz	128KB PseudoStatic 8 bit	512 KB EPROM	160x160 1bpp(1) no light	RS 232
Pilot 5000	DragonBall 16.6MHz	512KB PseudoStatic 8 bit	512 KB EPROM	160x160 1bpp(1) no light	RS 232
Pilot Personal	DragonBall 16.6MHz	512KB PseudoStatic 8 bit	1MB EPROM	160x160 1bpp(1)	RS 232
Pilot Professional	DragonBall 16.6MHz	1MB PseudoStatic 16 bit 80 ns	1MB EPROM	160x160 1bpp(1)	RS 232
Palm III	DragonBall 16.6MHz	2MB PseudoStatic 16 bit 120 ns(2)	2MB Flash RAM	160x160 1bpp(1)	RS 232, IrDA
Palm IIIe	DragonBall-EZ 16.6MHz	2MB EDO DRAM 16 bit	2MB EPROM	160x160 4bpp high contrast	RS 232, IrDA
Palm m100	DragonBall-EZ 16.6MHz	2MB EDO DRAM 16 bit	2MB EPROM	160x160 4bpp high contrast	RS 232, IrDA
Palm IIIx	DragonBall-EZ 16.6MHz	4MB EDO DRAM 16 bit 60 ns	2MB Flash RAM	160x160 4bpp high contrast	RS 232, IrDA, internal slot
Palm IIIc	DragonBall-EZ 20MHz	8MB EDO DRAM 16 bit	2MB Flash RAM	160x160 active matrix colour 8bpp	RS 232, IrDA
Palm V	DragonBall-EZ 16.6MHz	2MB EDO DRAM 16 bit	2MB Flash RAM	160x160 4bpp high contrast	RS 232, IrDA
Palm Vx	DragonBall-EZ 20MHz	8MB EDO DRAM 16 bit	2MB Flash RAM	160x160 4bpp very high contrast	RS 232, IrDA
Palm VII	DragonBall 16.6MHz	2MB DRAM 16 bit	2MB Flash RAM	160x160 1bpp(1) high contrast	RS 232, IrDA, radio modem
Palm VIIx	DragonBall-EZ 20MHz	8MB EDO DRAM 16 bit	2MB Flash RAM	160x160 4bpp high contrast	RS 232, IrDA, radio modem
Visor	DragonBall-EZ 16.6MHz	2MB EDO DRAM 16 bit	2MB EPROM	160x160 4bpp high contrast	RS 232, IrDA, special slot(3)
Visor Deluxe	DragonBall-EZ 16.6MHz	8MB EDO DRAM 16 bit	2MB EPROM	160x160 4bpp high contrast	RS 232, IrDA, special slot(3)
Visor Platinum	DragonBall-VZ 33.3MHz	8MB EDO DRAM 16 bit	2MB EPROM	160x160 4bpp high contrast	RS 232, IrDA, special slot(3)
Visor Prism	DragonBall-VZ 33.3MHz	8MB EDO DRAM 16 bit	2MB EPROM	160x160 active matrix color 16bpp	RS 232, IrDA, special slot(3)
TRG Pro	DragonBall-EZ 16.6MHz	8MB EDO DRAM 16 bit	2MB Flash RAM	160x160 4bpp high contrast	RS 232, IrDA, CompactFlash
(1) Can use 2bpp with special programming hacking
(2) Newer boards seem to use DRAM
(3) External slot; not compatible with PalmIIIx expansion slot

If you are interested in comparing the performances of these devices, you may want to visit the PocketMark section.