Yes, you would be able to run a 7850. Once the 3570k is in, those pci-e 2.0 will then become pci-e 3.0 enabled.
As for the chipsets check this link
. As you can see the Z77 chipset was designed with IB in mind, and thus supports USB 3.0 natively (along with some other minor details), whereas the P67 was designed for SB. Plus the Z77 allows you to OC you're GPU