This year , a 3.0 mobo isn't needed and I'm doubtful it will be next year. I bought a z77 instead of a z68 to help future proof.
You should say most people don't need PICE 3.0. Some with PCIE based SSD's make full use of the bandwidth today. Those who run 3 or 4 graphics cards in SLI / Crossfire could use the extra bandwidth.
To pile on what others commented, you will be just fine running the card in a PCIE 2.0 slot and you will see no performance degradation. Click buy, happy gaming.
If you like charts you can see PCIE 2.0 x16 is 1% slower than PICE 3.0 x16 on a GTX 680:http://www.techpowerup.com/reviews/Intel/Ivy_Bridge_PCI-Express_Scaling/23.html