http://www.hp.com has all you want for your iPAQ. IF it isn't there, it
doesn't exist.
Unfortunately, older HP/Compaqs doesn't have the necessary ROM to get
upgraded to PPC 2003. IIRC, you need to have a 38xx device or newer.
But check with HP's homepage.
Also, having flashed several 5450s running 128MB, I don't think it would
be to nippy after the upgrade. I've noticed that the 5450s get sluggish
after being flashed when it comed to things like tapping the start menu.