Just to follow up on this.
- Removed the card slot covers
- Removed the soundcard and put the TV-tuner into its PCI-E slot
- Took out the GFX, removed the cooler and applied a much thinner layer of thermal paste
Now the temperature stops somewhere between 90 and 96 degrees celcius in FurMark 1.8 and I can play Modern Warfare 2 with the temperature steady at around 60 degrees celcius.
Thus - problem solved.