If you can wait a little while longer, there are some new PDA phones coming out that have pretty advanced GPS functionality. The HTC touch diamond will have GPS and you can use your finger rather than a stylus.
Garmin is coming out with the Nuvifone, which promises to be more stable, since the HTC touch uses windows mobile (not known for stability).
And the iPhone 3G is due July 11 with GPS and 3rd party app support. No topo map software has been announced, but many believe it will come soon.
Not very many companies are selling stand-alone PDAs these days; most have moved to PDA/phone combos. Either way, these news PDA phones are leaps above what's currently out there, so it is worth the wait.