i'll put a strong vote in for my new dell axim x30. wifi for home use (sync, streaming mp3, light browsing from the couch, etc.) , bluetooth/cell modem (or an open hotspot

) for the road. completly unnecessary for me, but very cool.
videos kick much butt with the 624mhz processor. some of the highest benchmarks for a pda (if you're into that sort of thing). more of the 624mhz pdas are on the way, but dell is the first out and i love it.
price was very right...i got on at around $330 shipped, tax etc. that was about 6 weeks ago. keep your eyes on the deals sites and you should find a decent deal.
i use the bluetooth mostly for phone modem use, but you can use it to sync as well. i have had better succes (and obviously range) with the wifi. hit sync on the way out the door and by the time i get to the car, i'm golden.
only thing i don't like is the included case. why can't ppc makers take the hint and put a decent flip cover that can easily be removed like the sony clie's or the palm tungsten t3. i hate carrying a brick in my pocket. right now i don't use a case because of the extra bulk. to each their own.
as far as backwards compatibility....i don't think a and b are compatible right? i know b is backward compatible with g...just can't remember about a.
good luck, have fun.