if it's a PCIe 2.0 slot, then it will fit a card which supports 3.0. As far as I know there is no card which is ONLY compatible with PCIe 3.0. Most of them, most high-ends, are PCIe 2.0 + 3.0 supported. But you will need an Ivy CPU to make use of the 3.0 anyway.
So, a PCIe 2.0 x16 slot will be able to handle the new cards which come with 3.0 as well.
yes no matter what(unless it is 10 years old)even if it is PCIe 2.0 it is backwards compatible so no matter what it will work
yes a pci-e x16 slot can take in pcie cards of any size pci-e 3.0 is just the generation to which it belongs...it is just the latest bandwidth standards and has got nothing to do with its connector size.
Thanks guys you've been helpful
and for the motherboard I have a IPISB-CU (Carmel2)
Definitely upgrading though, going to start building my own computer