I thought about ripping my Ipaq 3970 and custom mount the screen on the dash, cheaper, easy and neater to implement. I abandoned the idea as it is not powerful enough for my needs.
My spec is :
Compaq Ipaq 3970
Flash card jacket / external battery
1GB IBM Microdrive
Garmin GPS 16 on a modified TOMTOM cable
I only used it for navigations since I can move it from car to car...maybe for mp3 too. Tomtom 2, destinator 2, alturion 5 are all slow when searching a road an a whole country map, reroute is very slow too...infact anything you do on PDA is slow. Tomtom is good but the PDA is just not powerful enough, cant do much with the speed either, 400Mhz is probably still the most powerful one.
Thats from my experience...
Im really amazed you can tolerate all those stuff on your 3630, I used to have one.