I was beginning to run out of disk space so since I needed to buy some new disks I thought I might as well try to see if using software RAID on the Mac Pro was worth it.
So I bought two 500GB Seagate Barracuda 7200.11 rpm drives and formatted them for RAID usage. I selected 64K block size – I think default was 32K.
Next I copied my boot drive to the RAID setup using Carbon Copy Cloner and made the new RAID my new boot disk. There seems to be some confusion wether or not you can boot from a RAID drive – I do it now so I guess you can.
The old boot drive was a standard drive supplied by Apple – a Seagate Barracuda 7200.9 160GB . I tested it using Xbench 1.3:
Disk Test 48.12 Sequential 86.64 Uncached Write 82.30 50.53 MB/sec [4K blocks] Uncached Write 82.89 46.90 MB/sec [256K blocks] Uncached Read 86.32 25.26 MB/sec [4K blocks] Uncached Read 96.43 48.46 MB/sec [256K blocks] Random 33.31 Uncached Write 11.15 1.18 MB/sec [4K blocks] Uncached Write 91.24 29.21 MB/sec [256K blocks] Uncached Read 90.87 0.64 MB/sec [4K blocks] Uncached Read 119.29 22.13 MB/sec [256K blocks]
Then I ran the same test on the RAID setup:
Disk Test 120.21 Sequential 182.63 Uncached Write 305.03 187.28 MB/sec [4K blocks] Uncached Write 323.33 182.94 MB/sec [256K blocks] Uncached Read 78.29 22.91 MB/sec [4K blocks] Uncached Read 362.56 182.22 MB/sec [256K blocks] Random 89.59 Uncached Write 31.76 3.36 MB/sec [4K blocks] Uncached Write 353.19 113.07 MB/sec [256K blocks] Uncached Read 170.70 1.21 MB/sec [4K blocks] Uncached Read 223.48 41.47 MB/sec [256K blocks]
So quite a bit faster at least according to Xbench. I also measured cold-boot times until the Quicksilver logo appears. Before: 2 minutes. After: 41 seconds. Nice! 
Now I just need to copy a lot of data from the old disks – unfortunately my brand new SATA docking station seems totally dead..