Sorry Jiří, that was a hasty answer from my part...
> I haven't used Slony by now. Do you have some advices or articles they
> may help? I have no ssh access at FreeBSD server box when PostgreSQL
> 8.0 server runs - so I hope this is no problem...
Slony is quite a heavy beast to install, I'm pretty sure now that I
think about it that you don't want to go that way.
What you probably want is to:
* make a separate schema dump + data dump;
* apply the schema dump to the old box and manually fix errors;
* possibly drop indexes/constraints so the data loading goes faster;
* load the data to the 8.0 box - theoretically here you shouldn't have big problems, but I'm not sure about that;
* recreate indexes/constraints if you dropped them;
The index/constraint thing is a concern if you have big amount of data, otherwise I wouldn't touch it.
HTH,
Csaba.