Hi Navarro,
Instead of installing the SSD in one of the hard drive bays, why not put it in the lower optical drive bay?
It is a doddle installing one there in a Nehalem (2009) Mac Pro:
http://macperformanceguide.com/Mac-InstallingSSD.html
Then you will still have the other hard drive bays free for additional storage in the future,or for a RAID set up.
One piece of advice based upon my own experience - buy a Mac friendly SSD like the Mercury Extreme range from OWC:
http://macperformanceguide.com/SSD-RealWorld.html
Although I have been, and still am, very pleased with my OCZ Vertex EX SSD, the drawback is that it is impossible to update the firmware when it is installed in a Nehalem Mac Pro.
Unfortunately, I bought my OCZ a couple of months before the release of the Mercury Extreme range, otherwise I would have imported one from the USA.
Initially there were "Sleep" problems with the Mercury Extreme SSDs, but they were resolved by a firmware update, and their performance is exceptional.
Regards,
Bill