PCI Express is a graphics card slot. The most recent version is PCI-e 3.0.
Basically the difference is bandwidth; how much data can be transferred through the PCI-e slot. PCI-e 2.0 has double the bandwidth compared to PCI-e 1.0. PCI-e 3.0 has double the bandwidth compared to PCI-e 2.0; 4x more than PCI-e 1.0.
The nVidia GeForce GT 120 is a very slow card so sticking that card into a PCI-e 1.0 slot will not decrease performance in any way. There are currently no graphic cards available that needs more than a PCI-e 2.0 slot. Maybe in another two years the really high end expensive cards will actually need a PCI-e 3.0 slot to avoid any performance bottleneck.