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You can get any old pci express card and put it in your pci express 1x slot providing it's open ended. If not you could always take a knife to the slot or a soldering iron apparently works better according to this guy
I've not actually seen any for sale, I just know they're theoretically out there. But yes unfortunately they probably will be a bit more expensive. Personally I'd be tempted by coupe69's suggestion of melting off the end of the pci-e x1 slot and putting in any old card, I would've done that instead of upgrading my board if I'd realised it'd actually work!
As for them playing nice, it depends which OS you're running. Apparently it can be done on XP and 7 but not Vista.
Obviously what actually happens on Vista is the two graphics cards do battle, and whichever wins disables the fan on the opposing graphics card, causing it to burn up and die in a flaming ball of Fail.
