I no longer have a 1,1 Mac Pro, having upgraded in 2008 to a 3,1 Mac Pro which is also showing its age compared to the 2009+ models. Perhaps my experience might help you.
However, in an attempt to keep it current enough to continue using it, I did upgrade the video and switched to SSD drives. I was very pleased with the perceived performance boost realized by simply swapping a SSD for my boot HD ... but, of course, I wanted more.
With the known speed constraint of the SATA II interfaces in all Mac Pros, I looked for PCIe SATA III solutions for driving my SSD and also purchased a Crucial M4 6G drive. I found there were problems with booting from PCIe with the 2008 Mac Pro, but there were some boards which claimed to have solved that issue. After trying several, I settled on a Velocity SOLO x2 which initially did not boot due to a conflict with another eSATA card I had for my external RAID-5 backup drive, but I have since removed that eSATA card and use the second SATA port on the x2 card for that function. It now boots (although there is a small delay due to it being consider an "external" disk), and is really quite fast, showing speeds of 260 MB/s write and 506 MB/s read.
I also tried something simpler ... using the 2 spare motherboard SATA ports, I mounted a pair of matched smaller 3G SSD drives from OWC on a bracket in the optical bay and configured them as RAID-0 for more speed. I am seeing 307 MB/s write and 311 MB/s read from that array, and of course no booting issues at all. You could also use the sled bays for this if available (mine are occupied with a SSD Windows boot disk, a pair of 1TB hard disks in RAID-0 for OS X data, and the original 500 GB hard disk for Windows NTFS data.
The above speeds were using the BlackMagicDesign "DiskSpeedTest" program.
I also measured with the AJA System Test (4 GB, and 1920x1080 10 bit parameter settings) with the following results:
PCI SATA III with M4 SSD: 260 MB/s write, 506 MB/s read (same as above!)
RAID-0 SATA II with OWC: 496 MB/s write, 505 MB/s write (wow .... much faster!)
These are both popular speed tests as seen on these forums, and there may be some algorithm sensitivity to the internal SSD controllers which accounts for the differences. I am currently running with the RAID-0 configuration.
Anyway ... that's my story!
Good luck with your upgrade and report back your results.
-howard
PS: if you elect to use RAID-0 for speed, it is good practice to have a reasonable backup system running.
