You can safely use K800 R1KG001 firmware version because it performs well and i did not experienced any bugs.
I also have SE K800i and i'm satisfied with it.
Remember to use only original firmware and not modified versions unless you know for sure what has been modified and it has been tested before by others with no problems.
If you want to change something to your phone you can play a little with some audio drivers, change some fonts, but I think Sony Ericsson know very well what they are doing and you don't really need to change anything as long as it works well.
And the most important thing is to read some threads and make your homework properly before you decide to change something in the phone files system. I think you already know that any change made in the phone's software will void the warranty.
Remember one thing. Mobile phones are not HI-FI audio-video components capable of very loud noise and high quality video playback, because this king of capabilities require lots of processing power in terms of processor, speakers and display resolution. If you want HI-FI standards use a HDTV or a dedicated HI-FI audio processing unit with some good speakers.