The i950 is noticeably better, although the i850 is no slouch,
especially since it only uses four colours.
Yes, there are two extra ink tanks in the i950, for a total of six
colours. They are photo magenta and photo cyan, which are lighter
colours which provides finer detail and shading in photos.
Have a look at this review to see a comparison of various 4 and 6
colour printers:
http://www.tomshardware.com/consumer/20030509/index.html