The PCIe x16 slots support auto-switching. When one expansion card is plugged in, the signal will be a PCIe x16 link. If cards are plugged into both slots, the x16 signal link will be split between both slots as x8 link speeds. This unique configuration will allow you to continue using your older expansion cards while also giving you a test bench for newer technology.
Remember that you can always UPGRADE the standard components in your system configuration! Be sure to use the link above to configure your system to meet your specific application needs.