I'm still using my Tungsten T3 for contacts, todo-list, calendar and even memos. I could probably use the N800 for memos, it's just a way of writing notes after all (and when I get my BT keyboard I'll probably do just that), but in my opinion the PalmOS versions of the three first PIM apps are much better than e.g. the GPE equivalents.

But I use the N800 for everything else!

EDIT:
However, if you do plan to migrate (which was you real question after all), then you can export the PalmOS contacts (AddressDB.pdb) in vcard format (just send it as bluetooth or infrared to some computer, I sent it through BT to my laptop). Now, if the vcard file can be properly imported into the N800 contacts database, that's the question.. I'm not certain about that. Or any of the other 3party contacts applications. In any case it's often necessary to massage the vcard file a bit, and I'm also worried about Nokia and the 'only one entry allowed in vcard file' syndrome (e.g. as in all their phones).

How to export memos I'm not sure, I haven't ever done that. Must be possible though.