I'm running this same machine in the 2.8 version with four hard drives and two optical drives and two added PCIe cards, about the same set up you have. My first concern is just where you would find the additional space in your machine to add more hard drives and why you would want to put such an additional load on the power supply and cooling system. You're just asking for a power supply failure. Consider a Mac Pro RAID Card and Xserve RAID Card to solve your problem. Here's some help on them: http://support.apple.com/kb/ht1346