It's good to see someone finally finding a use for the x1 slots, but if you want a cheap graphics adapter, go for a plain old (very old) PCI geforce MX4000 or a radeon 9550.
Like bourgeoisdude said really they're hard to find. And they are all restricted by the PCI bus, PCIe offers a bit of speed in an area that really needs it.
When and FX5700LE or FX5600 or GF6200 perform just as well (err poorly) as an FX5200 or R9250 because of the PCI slot, then you know you're limited. Of course neither of these card (the PNY Quadro or the Matrox) would perform any better, so like I said it'd be nice to see something like an X1300 or GF7300 on the PCIe 1X slot. And for the price really I'd say just replace your MoBo, heck even an integrated GF6100/6150 or X200 probably outperforms those PCI cards and those PCIe cards.