Well the video card is a huge upgrade especially if you run 2 screens..... iit'll also make a speed difference in OSX!!
The speed difference.... well it'll be faster!! Remember that it wasn't so long ago (beige G3's) that Apple was increasing speeds only in the region of 30Mhz to 60Mhz, so I think it'd still provide a handy difference in speed!! Remember also of Apple's double the RAM offer!!
That said, the PowerBook 800MHz is still a dandy machine (I own a TiBook 500Mhz, and it's no real chore working on it!!) so if the price difference is quite huge and you're on a budget go for the 800Mz..... , if the difference is really small... then go for the faster one!
Either way, I'm sure you'll be blown you away!😀