Assuming that WM5 is available for your PPC, you should read up on the changes (mostly negative as far as most folks are concerned) from Mobile 2003 to WM5. Many folks who upgraded, at least at the beginning of this year, regretted their decision.
I have both a WM5 PDA phone and a Dell WM5 x51V PDA. I think the phone's great. I detest the PDA.
You can't for instance, use ActiveSync over WiFi.
On many WM5 devices, Bluetooth is missing some features that it had in 2003. Like the ability to use a BT headset to listen to music.
Many Windows Mobile programs don't yet run under WM5.
Other stuff that may or may not be important to you.
YMMV, depending on the specific features of HP's version of WM5.
I suggest going to one of the HP PDA web sites and reading the Forums, so you'll be aware of what the issues are and whether they are important to you.