The 8600 GT needs a PCI Express (not AGP!)motherboardinterface and a 350 Watt power suppl built in. I don't know about the Siemens PC specification, please check in the manual if you have a free PCI-E slot on the board and the pc's power supply can deliver enough power
I just bought 8800GT, I have the same Mobo and it has PCI-E. I also bought new 500W PSU, how ever computer won't start, It does not give the power on good signal... Old card 7500LE works fine though. And yes I have attached the additional PCI-E power cable to it..