In case anyone is still interested in this matter, I am posting a short update. I have found my permanent solution to this matter and it is called Matrox P650.
Not only does it offer dual DVI outputs and desktop sharpness is increased over the Radeon, it also works perfectly (even better than the Radeon) in a HTPC setup using my Sim2 HT300+ and it does not freeze the system when Hyperthreading is activated.
I have had the Matrox P650 for ten days now. During those 10 days I have thrown each and every stability test at it I could find. On the exact same setup where the Radeon would freeze the system instantly, the Matrox performed flawless, as did the Geforce cards by the way.
You guys can say what you want, that my system is broken, whatever, the fact remains that this system works perfectly with either a Geforce or a Matrox video card present but freezes with at least 3 different ATI cards I tried.
Ati still denies that there is a problem but hey ... I donÂ´t care anymore, the Matrox P650 is even better, at least for desktop and HTPC use.
Thats it for me then :-) At least for now.
Thank you all for your help!