Currently in my cab I have an AMD X2 2.0 ghz with 2 gigs of ram. The currently gameplay on Soul Calibur through the onboard video was almost full speed, but just slow enough to make me consider putting a video card in (unfortunately, the PCIe went bad on this board, so I can only use PCI video cards). Before the PCIe went out, I was running an X1650 with 512 vram. It ran the game perfect.
My new main system is an AMD Athlon 64 X2 +6100 (dual 3.1 Ghz) with 4 GB RAM and a 1 GB video card, but I've yet to try NullDC on it.