Slot 4 is PCI 2.0. Slots 6 and 7 could be PCI 2.0 or PCI 3.0. It will depend on the processor users are using. Slots 6 and 7 are connected directly to the CPU. If users are using a processor such as E3-1220, the processor is only able to support PCI 2.0. If users are using CPU such as E3-1220 v2, the processor supports PCI 3.0.

http://ark.intel.com/products/52269/Intel-Xeon-Processor-E3-1220-%288M-Cache-3_10-GHz%29
http://ark.intel.com/products/65734/Intel-Xeon-Processor-E3-1220V2-%288M-Cache-3_10-GHz%29