Depends on what you're adding. I prefer P55's with more than 1 PCIe {x16}, you cannot CF/SLI nor add a PCIe x4 e.g. RevoDrive, etc.
1 x PCI Express x16 slot, running at x16
(The PCI Express x16 slot conforms to PCI Express 2.0 standard.)
4 x PCI Express x1 slots
2 x PCI slots