If it has a CF slot then that's even better - the transmitters stick out less, are cheaper and have a smaller chance of breaking (because they don't stick out so much!) but I didn't have an option with my iPaq...my model only has the SD slot so I had a look around (and decided NOT to buy one off of eBay...) and found one that has worked for over 2 years, without a hitch - the software is quite good too!
http://www.ebuyer.com/customer/produ...duct_uid=89714
I think it's gone up in price since I bought it though...maybe they've changed some of the insides?