I had to do another reboot because it crashed again. Right after reboot I did a 'free -m':
total used free shared buffers cached
Mem: 256 252 3 0 1 18
-/+ buffers/cache: 232 23
Swap: 511 59 452
Not much left is there? I'm not sure what the ISPConfig specs are concerning memory but I figured that 256M should be sufficient for a simple server running a couple of minor sites...