The Individual Answers
The i860 replaces the i850. Technologically speaking, they are very similar. Motor speed, quality, drop size and ink cost are practically identical. But the i860 comes draped in new modules: a 10x15 cm paper feeder in addition to the A4 one, a paper analysis sensor and a second black cartridge. The first one contains pigment-based black for text printing and the new one a dye-based black for photos. This means the i860 does not have to add in cyan, magenta and yellow to make black for high-definition photos.
Out with the C82, in with the C84. Epson has used the occasion to change the ink and cartridges, thereby gaining a few months before the compatible product manufacturers catch up. This is one of the (unofficial) arguments, but not the main point. The new ink is much more resistant to moisture, air and weather. Its chief advantage is that, even with clammy hands, you can take and hold the paper as soon as it is printed and the ink will not smudge. It's a fact, it works and it's the only one that does. Another good thing about the C84 is its price. While the Stylus Color 880 (late 2000) and its successors (C80 and C82) shipped at about the same price as their rivals, the C84 is half the price of the Canon and HP.