I went into a 'high street shop' and asked the same question as I was looking for a new printer at the time. There was a Canon rep there and he said that the 5200 has a smaller ink drop size...not sure how accurate this is or whether it is worth the extra money, considering the 4200, as pre1twa said, gives excellent prints anyway...
I do remember reading something about chipped canon cartridges on these forums, but never followed it up, maybe someone can shed some light.