I think so. Many x31 users did that.

I tried my 2631 (Dock I) with an old pci-video card (TNT2), it works well. The card is not full-size, still space left (2-3 cm in length) after plug the card in my dock. I think for most of the PCI-card it should work. I am thinking about buying a 9200/9250. BTW: Why Dock II? It's much more expensive.

not really, I recently bought a dock (2631) on ebay by 53 EURO (with 1 year guarantee). On ebay.com (US) it's even cheaper.
A PCI video card (ATI 9200/9250 + DVI/TV-out + 128 MB DDR) is around 50-60 EURO.

nobody interested?

what if you use a dock (e.g. 2631) and plug a pci video card (e.g. ATI 9200 PCI) which has DVI port?
very curious...