Sorry I am compelled to make some corrections here:
I have an HD 6970 running on a LGA775 motherboard; it's PCIe 1.1 and it works great.
PCIe (so far) is backwards compatible. The major differences are bandwidth and power rating.
PCIe 1.0 and 1.1 are 250MB/s per lane in each direction (that's the x16, x8, x4, x1 you see in technical specs)
PCIe 2.0 and 2.1 are 500MB/s
so a PCIe 1.1 x16 slot is basically equal to a PCIe 2.0 x8 slot in terms of bandwidth.
Tom has a great review that can answer more of your questions:http://www.tomshardware.com/reviews/pci-express-scaling-p67-chipset-gaming-performance,2887-9.html
Quote from that review: "Enthusiasts on lower budgets will find a general performance loss of 2% to 4% when switching from an x16 to an x8 slot."
But it looks like you have the budget for building a beast of a computer, so have fun!