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Is it possible to run windows games with PlayOnLinux if I run a 64-bit linux?
I now run 32-bit linux and just got fallout3 working. (with some help from this forum)
Will Fallout 3 still run if I install 64-bit ubuntu?
And do I need to do something special to make it work?

Yes, you can run PlayOnLinux on a 64-bit system. I'm not sure if you'll see a difference in performance but if so it should rather be better than worse.

If you want to try it before replacing your 32-bit installation of Ubuntu you might be able to use a virtual machine and run PlayOnLinux on there. To create a Virtual machine you need a tool like »Virtual Box«. Just install it from the Ubuntu repository.