At the moment a PCI-e 3.0 slot is not required by any card. It will probably be nVidia's GTX 800 and AMD's Radeon HD 9000 series that will begin to be a little limited a PCI-e 2.0 slot and would need a PCI-e 3.0 to get the full potential. However, that only applies to the very highest end cards like a GTX 880 or 890 for nVidia and the Radeon HD 9990 or 9970 for AMD. In other words if you are going to be spending $600+ on a graphics card in 2013 / 2014, then it would be best to have a PCI-e 3.0 slot. How much of a performance loss will there be if stuck in a PCI-e 2.0 slot? Not sure, but it should not be very bad. Natural slower cards from those future generation graphic cards will not be limited at all by a PCI-e 2.0 slot. The slower the card the lower the chances of a bottleneck.
Having said all that, if you want to take advantage of PCI-e 3.0, then you need an Ivy Bridge Core i5 or Core i7 CPU because the PCI-e controller is built inside the CPU. The Ivy Bridge Core i3 does not support PCI-e 3.0. It must be a 3rd generation Core i5 or Core i7 CPU.
If you are not the type of person to buy really expensive graphic cards and you upgrade the CPU/motherboard every 3 or 4 years, then you really do not have anything to worry about.