Hmmm ... v0.82. Stone tools from the dark ages? Wow.
If you leave the PS.FIR from 0.82 on your card, then you get 0.82 as that file is CHDK (essentially). But if you try to install 1.2.0 by just copying files, lots of things can go wrong - especially if your SD card is larger than 4G ( i.e. dual partition needed for your SD800 autobooting).
The easiest way out of this is probably to download STICK
and let it set everything up correctly for you. For reference, pretty much ever other (more difficult) method is listed here : Prepare_your_SD_card
I'd recommend going to the 1.3.0 version rather than 1.2.0. A lot of big improvements have happened in the last year or so!Note :
newer versions of Canon Powershots use ps.fi2 a file rather than ps.fir ..
I'll check to see if that's borked on the latest updates for the SD800IS.
The current distro for your camera has a ps.fir file.