All the PCIe slots are backwards compatibility. You can put a 3.0 in a 1.0 and it should work. The only problem is that the bandwidth will be very limited and could cause a bottleneck with the newer cards. Good luck to you though.
Theoretically it should work. However some of the old PCI-E 1.0 slots have had issues with the PCI-E 2.1 cards, it's possible those old slots may have problems with the 3.0 cards. Nobody has really tested brand new video cards with those older systems so it may or may not work. If it doesn't work check the motherboard manufacturer's website for a BIOS update that addresses PCI-E compatibility issues. I'd say go ahead and order the 3.0 card, but be prepared to RMA it if you have problems and there is no BIOS update to address the issue.
Limiting bandwidth is one issue. Compatibility is another one. There has been reports that PCIe 2.1 aren't working on PCIe 1.0 ones. Same principle can be applied to 3.0 as well.
BIOS update could fix the issue, but it isn't always certain. Usually if the BIOS update for the particular motherboard are of before mid-2009, chances are that it won't work.