My own internal 4GB SDHC that I've been using since mid-2007 to boot from and for swap and dozens of other people's similar arrangements on both N800s and N810s.
Actually, it IS better to have all these on one big device, as the wear is then spread across more flash blocks
Anyway, if this was a real problem, we'd be flooded with N810 users with dead internal cards by now
ext3 has been shown to be the superior FS many times. This is a pretty good writeup on why. The article specifically mentions is lower power consumption. Always handy in a mobile device. http://en.wikipedia.org/wiki/Ext3