can u not add 2.5 +2.5= 5 not 3.5
If you want to mount a single 2.5" drive inside the Mac Pro sled slot, MaxUpgrades makes a special sled that I've used to mount "bare" Velociraptors (which are really 2.5" drives when the ice pak is removed). I've also used it to mount a single SSD. Here's the link:
http://www.maxupgrades.com/istore/index.cfm?fuseaction=product.display&product_id=180
I used a pair of the above sleds to mount dual Intel X25-E SSDs this past week. I got over 500MB/s sustained READ in a RAID 0 set. And the small random transfers were "off the hook."
If you want to mount two 2.5" drives in a single sled slot, TransIntl makes the ProDrive. I've used it with dual 7K HDD notebook drives and with dual SSDs and works great. It supports HW RAID 0 and RAID 1. However, since it uses port-multiplication to share one port with two drives, the maximum transfer speed for any pair is about 220MB/s. Here is the link:
http://www.transintl.com/store/category.cfm?Category=2746&RequestTimeOut=500