Hi,
My wife and I have used our Palms religiously over the past couple years for English-to-Spanish reference. The best dictionary I've found is from HarperCollins available here:
http://www.palmtop.nl/palm/collinsf.html It has 110,000 translations and is available for Palm, Symbian and WindowsCE platforms. (That's geek-talk for "it should run on your Jornada"). It's well worth it's $35 price.
Another dictionary tool I like a lot is for when I'm using my computer and I just need a dictionary. It's from UltraLingua.com and it's very good too. Simple and fast just like the Collins dictionary, except it's on your desktop. It's also about $30.
There are other dictionaries out there for the Palm... but the size of this dictionary beats all the others hands down. I wish it had more words even, but it's pretty great. Note that if you're a Palm user, you'll need to have at least 2.5 megs free for memory to install both English and Spanish translations - otherwise you can only install one side.
-Russ