the i7 will be a lot cheaper than the i9
but I think the i9 will have lotsa USB3.0 and PCIe 3.0 while some of the i7's mobos only have 2 USB3.0 and no PCIe 3.0
i say go for a i7 920 and overclock... after all, the leap in performance between the i7 and i9 will not be much.
If you really think you can utilize the usb 3.0 and pcie 6.0 then wait for the i9... it's only 2 months... make sure you save every buck you have in that time