Here some stuff that'll add an order in your head, regarding SE definitions.
SonyEricsson Hardware and Firmware Definitions! - Top Sony Forums

Firmware is region specific, if you use SEUS (Sony Ericsson Update Service) it matches automaticly the version that matches your phones CDA (what CDA you need and which language included in which firmware you can read in the main page on the left).

The latest w800i firmware, as far as I know is R1BC002_RED36