I'm going to take this from a Sandy Bridge context. P67 (Cougar Point) has 8 PCI Express 2.0 lanes coming from the chipset. These support up to 5 Gb/s each lane (.8GB/s unidirectional I believe).
The Processor itself has a total of 16 PCI Express 2.0 lanes for use with the graphics card(s). Each of these lanes provide 8 GB/s.
So if you plug your GTX580 in right now it will be using the PCI Express 2.0 lanes from the CPU itself.
Plugging in the PCI-E sound card, in your case the Essence STX which is a 1x card (although the card will fit into a 16x slot). To much knowledge ALL sound cards only utilize 1x. Think about it this way even with perfect bit-streaming through ASIO with DTS-HD Master Audio which is an 8 channel lossless format you are utilizing 24.5 Mb/s. Compare this with a figure of 5000 Mb/s for EACH PCI-E lane. (this can also be verified by the width of the slot)
On the topic of PCI-E 3.0. The extra bandwidth is accomplished by upping the frequency not by any difference in the physical slot. Plugging a 2.0 card like the GTX580 into a 3.0 slot would only get you 2.0 speeds.
Outside of parallel computing even the 7970 sees no real benefit from going to the higher bandwidth of 3.0, since cards have VRAM not THAT much information needs to be shuttled over the bus. This is not the case with compute on GPUs because data needs to be transferred from the GPU to the CPU often and at low latency.
Edit: Note that the Sand Bridge PCI-E controller is more efficient than that of X58. Looking at benchmarks x8 SLI on the Sandy Bridge boards will give x16 SLI on older boards a run for their money, sometimes even coming out ahead.Edited by TickleMeElmo - 1/26/12 at 9:47pm