Good move getting StarTech to take back the RAID controller. It only supported modes 0 and 1 anyway and you can do that with the Apple RAID software (if you must.)
You still haven't provided much detail, but here's basically what can be done:
As The Hatter mentions above, you can use the SSDs on PCIe for your OS and Apps. This will speed your system up noticeablly. Your User stuff and go on a different drive based on how you set up your storage.
I'm guessing you're on Snow Leopard (OS X 10.6.8) so if you really need fast storage, you can stripe a couple of drives as RAID 0. THIS IS A DANGEROUS MOVE! A RAID 0 has to be treated with great care because it can crash at any time and is NOT recoverable. But, it will double or triple your data rate (my first SATA RAID 0 used 4 drives and would move data at 300MB/sec.on a dual 2.5GHz G5) I got away with it for a couple of years, but I was the only one who touched that machine. In the end it crashed without warning.
Before you go the RAID route, take a good look at what you're actually doing and what it requires. Your system supports SATA II drives that will transfer at 80-100MB/sec if properly maintained. This is fast enough to support several streams of SD or a stream (maybe 2) of HD depending on the size and codec. SATA III drives are everywhere and will work fine, you just won't see the higher speed because the MacPro has SATA II.
If you are using Final Cut Studio, there are lossless digital intermediates available that reduce the processor overhead (at the expense of data bandwidth) so that simple edits in HD are practical. Extensive effects work and heavy use of filters (like Magic Bullet) are no-nos as the render times are staggering.
iMovie is an option as well, but the K.I.S.S rule (Keep It Simple, Sir...) since the newer versions like to do everything in real time and depend heavily on a fast video board and high clockrate processors.
Hope this is helping