I bought my N800 specifically as an upgrade from my trusty TX, and I can honestly say the TX hasn't been switched on since. I've got the Garnet VM installed, and have played around with it a little; one thing to note is that when running fullscreen, scaling is a bit ugly - 320x480 doesn't go nicely into 800x480, while at 1:1 resolution, there's a lot of empty space around the palm screen. The one major app that I would have used but wouldn't run for me is Pimlico's Datebk6. Another point to consider if you're a long-term Palm user is that the handwriting recognition doesn't match Graffiti. Although you can customize it, I've tended to stick with the onscreen keyboard.
I'll confirm what everyone else says about the limitations of the PIM software for Maemo; that said, it's good enough for me. I'm using GPE for contacts, and Maemopad+ for memos. I can offer scripts for converting from Palm dbs via pilotlink if needed. The main advantages come through the larger screen - as a result of this, Claws on Maemo beats any of the Palm mail clients hands down, indeed any handheld mail client I've used, and both Opera and MicroB really show up the failings of Blazer, Opera Mini or (shudder) Pocket IE for browsing.
The other major difference is that I stopped using my handheld as an add-on for my desktop, and now use it as a standalone computer in its own right. For example, I no longer sync with my desktop automatically every morning; the N800 downloads and converts my plucker feeds itself. Ditto for accessing files across the network; I no longer need to copy data across in advance, as I simply access my home PC directly over sshfs or openvpn from where-ever I may be. I find the N800 to be a vastly superior machine to my old TX almost every other respect, and the PDA functions, although somewhat limited, are good enough.
Finally, nobody else seems to have answered this - I've just checked, and I couldn't see any way to access either of the card slots from within GarnetVM. There's a way to do so in POSE (the old palm emulator) which is available for Maemo, although I haven't tested it.

NB. Apologies - didn't check the date of the original question; however, my comments may be useful, so I'll leave 'em here.