Sure ... the internal memory is fine for the internet, loading apps, and some of your data (incl a few songs) and some ebooks. Once you get into the N800 and exploring its true potential you'll find yourself filling up those future SD Cards, but for right now I think you will have a fine experience.
It's six-of-one/ half-dozen-of-the-other, so take this as a mere suggestion: N810s come with 2 gig internal, (in lieu of the N800 internal card slot). They use different (physical) size cards, keep in mind.
You will want a larger card(s) in the future, though, if you plan on storing a few feature length movies for trips or whatever.
The storage space goes FAST since the devices are good at so many different things.
If you're going for an n800 I'd definitely suggest getting an SD card (or 5) especially to begin with.
That 256 Meg isn't very much at all when you've only just got your tablet and want to experiment with everything that it can do.
Once you've got a nice size SD card, I also recommend booting from that SD card, so you can have much more internal space for all of those funky applications.
I find that having 24GB in my N800 to be pretty valuable personally. From work and ministry documents, to 10+GB of music, it pretty much works out really well for me. And considering that 16GB SDHC cards are pretty cheap these days, its not a bad idea in the respect of having a 32GB, totally customizable pocket computer.
I have 32gb in mine from day 1 (2 x 16gb), and find that the space is usually used. You can often pick up 2 and 4gb cards really cheaply, and being able to swap several could be useful. also if you want to store music, consider encoding the files in aac. It saves loads of space, and I have been unable to discern any audible difference.
I'm not into watching movies on a tiny (relative to a TV) screen, so I find 16 gigs or so to be plenty; I'm sure I would be happy with less. But sdhc cards are so inexpensive...
I have to say that life is better with an SD card. The transfer rate is much quicker when writing directly to the card via an SD reader/ writer rather than connecting the N800 directly.
Storage is overrated. like, who is going to look at all those pictures anyway. If there is something you'd really like to keep, attach it to a gmail email and mail it to yourself