How much free space should there be on the start volume? I have a MacBook 2,16 GHz with 3 GB RAM (white from 2007) and I wonder how much free space should there be on the start volume.

I have a 100 GB partition for OS X (10.6.5) and a separate data partition of 300 GB (however the user directory is on the system partition).

free space on the start volume:
30 GB after booting
22..25 GB after one day of use
goes down to about 22 GB after several days of use (I usually reboot, when it reaches 22 GB).

(there is also about 30 GB free on the data partition).

=> Could that be a performance problem or is that enough free space?

I have some large databases open in DevonThink Pro, mostly many pdf files open in Skim,  Safari seems to need quite some RAM and there are some non-native apps (TexMakerX and JabRef) which also get quite slow sometimes.
I've run for 3 years with no more than 10GB ever free on my 160GB disk, and quite frequently as low as 500MB free. Things seem to work just fine.

Aperture complains loudly if there's less than about 1GB free: I think that would be a sensible minimum to stick to.

Your system swap file does get recovered without a reboot -- if memory usage drops enough.

I find that Caches, particularly those of Safari.app and Spotify.app are a big cause of unnecessary disk use. You can rm -Rf ~/Library/Caches whilst the system is running without any bad effects if you're about to run out of space!