Good news, I 'm not with SL 10.6.4 and still have my Samsung Syncmaster 223BW.
I have a new mini DVI-to-VGA cable and it works.
In fact, I 'm not 100% sure that it was the cable, because it did not work the first time I plugged it into my macbook.
But, I noticed that there is two 1680x1050, 59.9mhz for my Samsung. So I selected "Activer la recopie video" and then "Désactiver la recopie video" (this is the only way I had to use my external display at 1680x1050). Then, I choosed the another 1680x1050, 59.9mhz. Reboot the Mac to try and, what a miracle, it works!!