What is the best upgrade I can do on my SE K800i to get the most out of it?

[SeaMonkey]

I have seen applications on the forum (SETool 2 Lite, XS++ and FAR) but don’t know what they really do. I guess the first thing I must do is upgrade/change my firmware. Can I upgrade it to K810, K850 or even better? Which application is best for that?

Yes you can convert K800 -> K810 but the tool depend on CID of your phone, so for start just install USB flash drivers and connect phone with XS++ and XS will create log-that log post here without IMEI number and will direct you to wright direction.
Here you have tutorials: USB Flash Driver Installation - Guide - Top Sony Forums
XS++ 3.1 Guide-Flashing, Customizing, Recovering - Top Sony Forums