I can't actually recommend a particular device, as I use an Axim X51v which Dell have decided to cease manufacturing (big shame, as this is a lovely device to use).
So instead, here are a few things that you should consider....
There's three main styles, Folding, Tablet and Sliding.
Onboard memory and then SD/CF options.
Most devices have Infa-red and Bluetooth, but do you need buit-in wireless or GPS?
Presumably you will be looking at Windows Mobile 5 or 6 if your wanting to run B4PPC.
Probably the biggest decision is how much are you willing to pay, devices can range from £100 to +£500
I also wouldn't rule out a smart phone, OK the screen is smaller but then you've got a PDA, camera and ...uhm oh yes a phone crammed in there. My friend has an MDA Vario, now that is a nice unit!!