Of course, I purchased a 12" G4 iBook in January...so I'm slightly jealous of the new iBooks (especially since a friend of mine purchased one as her first computer ever...and got it for the same price I did).
If it's any indication of what you can do--I use Final Cut Pro (and all it's little peripherals...except Cinema Tools...haven't really used it yet), the entire Adobe Suite (Photoshop, Acrobat Pro, and After Effects mostly...hoping to learn Illustrator soon), DreamWeaver and GarageBand--I have no problems. Final Cut did crash on me yesterday--but that was probably due to the fact that I was also running three or four other apps at the same time.
As long as you don't want to do hardcore multi-tasking (like using some of the above programs at the same time) the 800 mHz should be fine. However, I think the major difference, aside from processor speed is in RAM. The 800 has a 128MB set soldered in, as opposed to the 256MB set in the new iBooks. I put in a 512 for a total of 640MB, but having an extra 128MB would be REALLY nice.
For what you're wanting to do...either is fine...the new one should last a little longer (plus AppleCare and stuff, don't know if you have that option on the used machine).
Either way...welcome to the iBook clan! Forget those overpriced PowerBooks! (I just have pbook envy...)
