You can't unlock the CPU on that motherboard (which is okay). A single GTX 460 would be HEAVILY bottlenecked by the CPU. I know because I had it and was bottlenecking a GTS 250.
According to Gigabyte's website, that board supports all AM3 CPUs: http://www.gigabyte.us/products/product-page.aspx?pid=2442#
Update the bios and then upgrade the CPU to a Phenom II x4 955BE or 965BE--whichever comes up on a better sale in the next few weeks. You shouldn't have to pay more than $115.