Some games can use a dedicated card for PhysX simulation.
Other than that, I can't come up with anything that the second card could be used for (in terms of enhancing performance).
The difference between x1, x2, x4, x8 and x16 PCIe is the maximum throughput of the bus (how much data can pass between the card and the rest of the system in a given time).
Directron lists the following speeds:
PCI Express 1x 250 * MB/s
PCI Express 2x 500 * MB/s
PCI Express 4x 1000 * MB/s
PCI Express 8x 2000 * MB/s
PCI Express 16x 4000 * MB/s
PCI Express 32x 8000 * MB/s
* Note 1 - Since PCI Express is a serial based technology, data can be sent over the bus in two directions at once. Normal PCI is Parallel, and as such all data goes in one direction around the loop. Each 1x lane in PCI Express can transmit in both directions at once. In the table the first number is the bandwidth in one direction and the second number is the combined bandwidth in both directions. Also please note that in PCI Express bandwidth is not shared the same way as in PCI, so there is less congestion on the bus.
Update: Table above contains speeds for PCI Express 1.0 bus. For version 2.0, multiply all bandwidths by 2. For example a PCI Express 2.0 16x slot has a max bandwidth of 8000 MB/s one way or 16000 MB/s both ways.