First - welcome to Mobility Site! I am not sure that I am going to be a wealth of information but I will pass on the little bit that I do know.
I would look into a SDIO bluetooth chip - this is probably the only way to get that iPAQ to have bluetooth - but you will have to look about a bit for one that can hopefully offer you storage too or having bluetooth will be a little disappointing as you wont have room to store maps. I did a quick google and found a SDIO bluetooth
here another possibility may be (if you want GPS in the car) to look at
Seidioonline.com and see if they have a cradle for the rx1955 that will supply power to the unit and provide a wired port (kits are usually available with a wired GPS receiver as well) for a GPS receiver. I have used this solution from Seidio for my previous h5450 and my current hx2755.
There have been some issues with drivers for WM5 from various manufacturers for the SDIO solution (as evidenced in my quick search) but they all seemed to be the biggest concern early last year (2006) make sure you check the compatibility before you buy!
My dad has a rx1955 and as I will be seeing him this weekend - if he has come across any other solutions that work well I will revisit and pass on to.
Happy hunting and again - Welcome!