You need a special cable or adapter to use a USB device with the Dell Axim X50/51v. I found a better solution for paperless geocaching with a PDA was to:
1) Use a Dell Axim X50v (or a X51v) and upgrade it to LennySh's ROM of Windows Mobile 6.1
2) Get a SiRF Star III CompactFlash GPS receiver such as Holux GR-271, Globalsat BC-337, and there are several others available for under $30 used on eBay
3) Use GCzII Geocaching software
4) Install Google Maps (can be used with GCzII software)
5) Tether via BlueTooth to cell phone or via WiFi if available for Internet access
6) Recommended but not required: extended or mega-capacity battery, around $15-$20 in eBay
This does not require the use of ANY external GPS device so you only need one device, and allows for completely paperless geocaching. The software pulls caches located close to your current location, allows complete access to description, cache logs, hints, trackables, waypoints, Google Map images, and compass directions all from the PDA directly, it also allows adding field notes, posting cache logs, and everything else you would normally do from a PC WITHOUT needing a premium account or needing to mess with GPX/LOC files.
I know use this as my primary caching tool, it is so much less cumbersome than any other setup I have tried or seen others use and it works extremely well, the only issue is it is not weatherproof in any way whatsoever.
If you need any more information, feel free to ask...