i've got the 800i, it does work really well, and for the price in almost impossible to beat.

the 830 doesn't have many extra features, and is hard to justify the extra cost.

the 850, well that one does justify the cost, i think. in Aus. it comes with an extra lens, a 1.7mm (?) wide angle, so that's a fairly major plus (assuming it'd be useful). it also has an extra 2x zoom (making it 22x optical zoom), it comes with some capture software (the 800i doesn't), has 18 effects (depending on what that includes, this is a few more than the lower models), and it'll take crappy still pics too. it's also got more connections and slightly more buttons, and obviously if it has more buttons it must be better! :P

it doesn't have a higher quality CCD though, it's still just the 800,000 pixel job, so if all you are interested in is picture quality then save your money for a better mic or software and het the 800i.

my 2c