Just do not overwrite internal flash too many times and it will survive long enough. Of course, permanent swapping to flash not a good idea. But you will dislike how device works while reading/writing swap file too much anyway - it's awfully slow and so you will be only able to tolerate very light swap file usage anyway. In such cases only almost "unused" code or data will live in swap so writes are quite rare while RAM is still freed from almost never used code/data, hence giving more room for some useful things instead.

However let's admit I like n800 with 2 cards better in this aspect. You can both replace card (in unlikely case it wears out or just to install bigger one) and use 2 full-sized SDHC cards. And if you're really worried, put frequently rewritten data to external card(so you can replace it in the case), As well as you can do with swap file (after learning couple of things about Linux so you can specify things from commandline or startup scripts rather than use GUI dialog to do it for you).