Glad you found success! I also have my Catweasel working (with SID chips) in my X1000 now, with a 3.5" floppy.
I have my StarTech adapter in the upper PCIe 1X slot (closest to CPU) and that is the only one I tried it in. It worked straight away on first bootup afterward, so no futher fiddling on my part. I've got my slots pretty well loaded right now, with little room for adding anything else except maybe something in the Xorro slot.
Here's what I have in order from top to bottom:
PowerColor RadeonHD 6850 (single-slot card, same as written about by Trevor)
StarTech PCIe to PCI adapter with ethernet card
GigaByte RadeonHD 5770 (takes up two slots of space, so blocks next PCIe 1X slot)
Xorro slot empty (so there's antoher PCIe 1X socket inline here???)
Catweasel MkIV in PCI slot
SoundBlaster in PCI slot
Everything works. I have two displays going so right now I am looking at DOpus on one screen and MUIOWB on this one.
What I couldn't get going is three screens (with 9250 in PCI slot) though I followed Hans' suggestions when I tried. Not sure I will pursue this given that speed of Blender interface with wazp on the 6850 seems comparable to me to speed of Blender interface using warp on the 9250. In other words it's not as fast as I would like on either, but I expect we will get there eventually. Favoring warp on the 9250 is that I can see particles in my animations with that, and I cannot see particles without rendering on the RadeonHD card. Besides, the Catweasel is happy.
Always good to keep your catweasels content.