Paul,
From the Epson America website:
Model V850:
Epson Perfection V850 Pro color scanner
8" x 10" Transparency Unit (built into lid)
Eight film holders: 35 mm slides, 35 mm film strips, medium format 6 x 20cm and 4" x 5" (two each)
Film Area Guide
Scanner Software installation CD-ROM
LaserSoft Imaging SilverFast SE PLUS
X-Rite i1Scanner with reflective/transparent IT8 targets
Setup poster
Hi-Speed USB 2.0 cable
AC adapter and power cable
Model V800:
Epson Perfection V800 Photo color scanner
8" x 10" transparency unit (built into lid)
Four film holders: 35mm negatives, 35mm slides, 6 x 20cm and 4" x 5"
Film Area Guide
Scanner Software installation CD-ROM
Setup poster
LaserSoft Imaging™ SilverFast® SE
Hi-Speed USB 2.0 cable
AC adapter and power cable
So: An additional set of film holders, a higher grade of SilverFast software (useful, but the Ai 8 Studio version would be better still, available for a reasonable upgrade price from the version supplied with the V850), X-Rite scanner calibration software and materials (not needed if you use SilverFast Ai 8 Auto IT8 process - much easier and faster).
The main operational difference is the provision of "advanced optics" technology in the V850.
For the $200 price difference between the two models, once you are getting into this and plan to do a lot of work with it, were it me I'd buy the V850. That $200, sad to say, is a couple of bags of groceries here in Toronto these days, but you may appreciate value of the extra features over time. I would also recommend getting and using the Fluid Mount Accessory.