Now I want to reinstall the distro and so I have to download the ISOs for myCPU model. But I can't find them. Or better, I don't know which is the correct one (IA64 / i686 / ... ??? )
you _do_not_ have to install your entire system just to make use of the amd64-architecture! just install the kernel-image available (preferably the latest 2.6.X one) for amd64 and update the few programs dedicaede to a special cpu (mplayer is the only one occuring to me). if you are completely new to debian and linux you should read f ex http://www.debian.org/doc/index.html.
Does Debian support my CPU model?
yes.
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"Schon vor dem Come-Back von Modern Talking wusste ich:
Dieter Bohlen ist der Preis der Freiheit."
Heinz Rudolf Kunze