"So I must do something wrong or I am missing something!"
The problem may be in the way that you have configured your hardware. Maybe you have not allocated enough video memory. Check your BIOS and see how much memory you have set aside to be video memory. This may have been set to a low value when you added 512M of ram.
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Be prepared. Create a LifeBoat CD.
http://users.rcn.com/srstites/LifeBo...home.page.html
Steve Stites