PCI Express 3.0x16 Card work on 2.0x16?

Like the others have said: Yes and you'll see practically no performance loss with a PCI-E 2.0 motherboard.
One point I'll make is that some motherboards can have issues with new cards, so I'll suggest you update the motherboard BIOS (if possible) before installing the new card, even if you purchased the 'board recently it's possible updates are available.
Bear in mind, flashing the BIOS is a potentially risky operation, be sure you know exactly how to proceed and as my old trainer was fond of saying: "if in doubt, do nowt." (If you do not know what you are doing, do nothing).
 

KENDAWG2000

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Dec 6, 2014
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Thanks for responding, yeah I don't know, so I guess nothing is my option :p