Crystal,
Sure, any PDA that runs Windows Mobile will work, some better that others and then you need software. There are dedicated machines and there are general purpose. Dedicated ones include Symbol's MC 1500, ScanPal2 from Metrologic, Uniden makes a model, many others etc.
Software comes with each machine to create a text file to import into RMS. Fancier software from New West Tech
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or 2B Inventory
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gives even more functionality.
Others have used a Palm Pilot or whatever they are called today with a piece of software from Stevens Creek called OnHand.
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Personally, I recommend a wireless laptop and a scanner. Load up RMS onto it, put it on a rolling cart, go into SO Manager | Inventory | Physical Inventory and start scanning and counting. Laptops can be cheaper than the dedicated PDA's
Just be sure to set up the wireless correctly secured, WPA2, only allow certain machine's MAC addresses, etc.