To use SAS drives, you need to keep the Apple RAID card.
The Apple RAID card is limited to a maximum of 2.2TB per drive. If you install a larger drive, the rest of the drive (beyond 2.2TB) cannot be accesssed.
The Apple RAID card is limited to SATA II (as is the Mac Pro with no RAIDcard).
The data stream from a single Rotating drive or RAID cannot overwhelm the SATA II bus speed, so having SATA III is not an advantage for rotating drives.
The Apple RAID card precludes the use of SSD in the main drive bays, but you can put one in an optical bay with the ODD ports and some chaeater cables, or install one on a PCIe slot card.
If you have somehow avoided issues with the Apple RAID card consider yourself lucky probably.
SSDs, 10K WD VRs (180MB/sec) and the large latest enterprise 7.2k drives using software RAID are all fine.
The RAID card for data only and RAID5 and boot the system on independent drive - and bus.
WD 10K VR 250GB $100 / same as say Samsung 128GB SSD / 1TB WD Black - for your system
4 x 2TB for your data (I tend to shorten the partition volumes if I want to force using 1/2 the drives faster tracks).
10K VR 1TB are also very affordable and solid units.