I believe a x4 slot should be the same as a x16 slot physically. You can put any card you want in it. It is going to limit the high end cards some but it should be ok for low end and mid range cards(let's say under $200 or so.)
IMO, no. There are no "decent" PCI-Ex1 graphic cards. As far as I know there are no PCI-Ex4 graphic cards. If the PCI-Ex4 slot on your motherboard is open-ended, you can just use PCI-Ex16 graphic cards, though it won't run as well as in a PCI-Ex16 slot (depending on the card).
Dual Vsta boards are X4, they are old boards now and I know of people running 4870's on them with a modded bios. I would think that would be the issue here, support for the GPU in question.
I recommend the ZOTAC GeForce GT 610 PCIe X1 over the 520 because the 610 has HDMI and DVI connections plus one GB RAM. The 610 has DirectCompute support and runs anything I use. While I am not a big gamer, I do use high resolution displays. Best Regards, Christopher