If you aren't too serious about gaming, you could go with either i3-3220, i5-3470 (or whatever else may be most cost-effective at your preferred parts store) or i5-4570.
If you are tempted to go Haswell (the 4xxx models), you may wish to wait a few months longer for lower-end chipsets to get released.
The PCI Express Revision(s) are determined by the motherboard's chipsets.
On the GIGABYTE GA-G31M-ES2L the PCI Express x16 socket is connected to the North Bridge's Intel G31 Express Chipset that uses PCI Express Revision 1.1.
The PCI Express x1 socket is connected to the South Bridge's Intel I/O Controller Hub 7 (ICH7) chipset that uses PCI Express Revision 1.0a.