[Problem]
I built my new box on the base of a Asus A8R32 MVP Deluxe motherboard. Since the gfx board and RAM was not delivered yet, i just put in a single DIMM (Single RAM only works on RAM slot B1!) DDR 266 and a PCI gfx board, to see if it would actually boot up or if the delivered motherboard was broken.
[Solution]
The Asus A8R32 MVP Deluxe does not boot from PCI gfx cards. As soon as i had the PCI-E gfx card and put that one in, it came up like a charm. Its a crossfire-ready board, so make sure you put the gfx board in the blue (primary) PCI-E slot. As mentioned above, also doublecheck that the RAM - if you are using one single dimm, that is - is put into the first slot of the second bank. Its called "B1" and is the blue slot on the B-labeled ram-bank.