that's wat i was actually just thinking.. so it's the primary that does the work and I would run the 2 19" off the secondary correct? I'm interested to see how it goes with COD4 as this is what I play most at the moment. I should be able to get a decent frame rate if I drop the graphics settings, but if it works I would like to upgrade the primary card, just need to see if a 4870 will fit in my case... does softTH work with crossfire? if it does, I could possibly continue to run the 2x 4670's on crossfire, and buy another el-cheapo pci-e x1 card as my mobo has 2x pci-e x16 and 4x pci-e x1
I don't believe that Crossfire works with SoftTH.
You can either run two monitors off of the first card or the second, depending on your preference I believe (it makes more sense to run two off of the primary card, though).
You don't want to get a PCI-E x1 card, because the major bottleneck for SoftTH is the speed for the connection between the two cards. From experience I know that running a PCI card it was impossible for me to get more than 20 fps (regardless of what was being rendered, 2d or otherwise). Try it out with your two 4670s, and then if you feel you need a boost in speed upgrade one of the cards.
Or, better yet, if you think the speed is fine, open up your case and drop one of the 4670s into a PCI-E x1 slot and see what your bottleneck would be.
Other than that, the second card does absolutely no rendering so speed of the card itself isn't really an issue.