I use an HP iPaq hx2415 with my Valeo 22wi and it works great most of the time. Battery life is good - I can generally get through a long shoot on one charged battery.
The screen on the 2415 is really nice. It's 320 x 240 but looks great.
I found that most of the iPaq proccessors in the mid-to-high end are about the same. The differences come in the amount of ram. My iPaq only has 64 megs which is fine running Windows mobile 2003 but everything I've read states that you really need 128 megs to run Win mobile 5 which I would like to upgrade to.
My only real issues with the iPaq is it unpredictably drops the blue tooth connection. I usually keep the iPaq running and in a front shirt pocket or in my waist bag and some days it stays connected perfectly and others it drops the connection and has to be rebooted. There are times that after I've seen my first preview I just start shooting and say the hell with it.
Here's the specs for the hx2415 - you might be able to use this as a jumping off point comparing new iPaq's:http://tinyurl.com/2acgma
Anything you figure out regarding a newer iPaq I'd really appreciate you passing along.