Quarter of the speed does not mean that it will be quarter of the performance as the GPU will not always use the entire bandwith of the PCIe slot.
With PCIe 1.0 there used to be a significant difference in performance when moving down to x8, x4 etc, however with PCIe 2.0 and the coming PCIe 3.0 this is not such an issue anymore, Here are some Charts and sources for you:
have a read of that it explains it all, the difference between x16 and x8 for PCIe will be barely noticeable in a game
PCIe 3.0 x16 = ...................... = 16GB/s (no GPU out there need this speed yet)
PCIe3.0 x8 = PCIe 2.0 x16 = 8GB/s
PCIe3.0 x4 = PCIe 2.0 x8 = 4GB/s
It tells you that on the web page.also as you can see the pcie2.0 x8 is limiting the card but only by 1-4% so in other words PCIe 2.0 x8 is not an issue at all. so you will be fine with your current motherboard.
That should explain everything there is to know about PCIe versions and speeds and how they will affect your performance