I have done some research a month ago, and come up with this baby:
MSI NVIDIA 6800 ULTRA w/ dual head PCIX
What is so special about this board (beside chipset and 256mb) is the PCIX communication interface. It is, as far as I have been able to find out, THE future. Way faster than old AGP.
Of course, then you need a motherboard that is compatible, which would take you to a BOARD MSI 915 PNEO II PLATINUM .
And, as you will need lots of fast Ram, I recommend 2*KINGSTON 1024 DDR 400 CL2.
Ah, do not forget the hard drive, because ArchiCAD9 is REALLY slow opening, so my advice is RAPTOR SATA 74.00 GB, which apparently is as fast as a SCSI (you should see those babys opening windows: Wham!)
This has brought you about 2000€ lighter, and we haven't even talked about interface (pen, monitor, the works).
And in about one year, this all will be a pile of junk...