JaXXoN: yes, you're right. The board is equipped with PCI as well. I didn't know that the 8400 was available as PCI. Doing a little search I found such a model, but for nearly twice the price I paid for the PCIe version!
Furthermore, I can't find a model without a cooling fan.
But I also found a "GeForce NVidia 6200 256MB DDR2 64BIT AGP8X DVI TVO Heatsink" by Gigabyte for an acceptable 25 Euros. Now, if I am not mistake again, the 260 driver supports the 6200 and the 8400 at the same time, doesn't it?
For most of the work that's being done on that machine, even the FX 5500 is efficient enough. I only bought the 8400 because I needed a PCIe card without a cooling fan and because it was a special offer. So, if the 8400 and the 6200 can work together with the same driver, I rather get the 6200 AGP, because it is only half as much as the 8400 PCI version.