I had that battle - but the other way round. PDA first, then found TomTom.
After weeks of stomping around electronics shops I purchased a PDA direct from HP, through their web site.
There was a "chat with technician" option that suddenly popped up whilst I was browsing.
HP asked what I wanted the machine to do - then steered me away from the machine I was looking at (a 2900 series, iirc). They said the processor and memory, although higher spec. than the one they suggested, was not ideal for the job and was more expensive.
They talked me into an HX2495 model which runs TT Nav6 extremely well. It's not a phone (unless I'm in a wi-fi hot spot, at which point I can use Skye) but does have wi-fi, bluetooth and infra-red.
I've added TWO 4 Gb expansion cards to it, one 4Gb SD card and one 4gb flash card. I bought the two cards on ebay, for about $68 each including shipping). I also bought a Holux GPSlim360 GPS unit (20 channel, works indoors, and communicates superbly with the pda).
I've seen the hx2495 on ebay for under $200 - so for around $400 (GBP200ish) you can have an 8Gb pda with GPS.
A computer guru friend of mine saw the kit and fell in love with it - works a treat, very neat and very very simple to use.