no, traditionally the new chip will be ~$1-2 cheaper than the previous version, and then the price will go down slowly on a price per performance basis until stock runs out. If the Ivy Bridge chips are such bad overclockers as the reviews say so far then there may be a high demand for SB chips for a little while yet.
Still, there will be several people out there who 'must' have the latest chips and will be selling their old 2500k and 2600k chips, and will sell them for a decent price.
No, it will not make single PCIe3 GPUs run better. What it will let you do is run high end GPUs with 4 or 8 lanes without bottlenecking, where they would bottleneck on 8 lanes with PCIe2. This means you can run multiple cards in SLi or xFire without issues when there is not a duel 16 lane option available.