Well, no big deal! I just followed the recommendation from naquaada, who advised to try Onyx to repair permissions. I did that by opening Onyx with admin priviledges, otherwise it won't work and go to "Automatisieren", and then use the following options:

repair permissions
peroidic scripts
dydl's shared cache
Mailbox index
and all the following options ticked. (system cache, user cache, Fonts cache, protocols and crash reporter, temporary files, recent documents and web browser cache.

Give this a go, then do a restart, and go to system preferences panel "Monitor" and change the resolution there to the wanted value.

Then it gave me a blue screen and no option other than a holding down power button to hard shutdown the machine.

You might be able to spare the hard shutdown if shortcut to shutdown the computer will work (ctrl & eject key then return)

Try that and kindly report if it helped. Sorry I had to translate, but I am on a german localization.

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MacBook Alu 2,4 GHz, 4GB, Seagate 7200.4 500GB, iPhone 3G 3.0
Shuttle SG33G5, Intel C2Q Q6600 2,4 GHz, 4 GB, Samsung HE753Li (retail Leo 10.5.6), Seagate Sata for Vista, PX-810SA, Nvidia 9600 GT 512MB

SG33G5 runs Retail Leopard 10.5.6 (vanilla) thanks to boot123!