As for storage capacity, you can be sure that the mini & micro sd cards will get bigger as time goes on. Already, the biggest current SD is 16GB, mini SD is 8GB, but 16GB should be around the corner. So I wouldn't count out the n810 simply because of the memory card size.
However, my biggest gripe with the n810 is the fact that the internal flash is permanent. And I'm not sure if 2GB is big enough. What I'd like to do is have 1 large SD card internal, and boot the device from that card. On top of that, I'd like to load up alternet OS distributions on it (I'm thinking of porting over one of the distros available for the Sharp Zaurus such as pdaXrom). Or load up a full Debian install on the internal flash. So thats the reason I picked up the n800 (plus the lower price). Also, I'd rather use an external GPS module so it can be located up by the car window (to get better satellite reception) rather than an internal gps chip (the minivan has some sort of solar filter imbedded in the windshield, and there is only one spot by the mirror where a gps receiver works).

So, if you don't think you will need the keyboard, you can go for the n800 for now, then a year later pick up the n810 when the price comes down (after they introduce the n900). Or, when the new model comes out, you can more easily justfy picking it up since your current one is 2 generations behind instead of only 1 :-)