I have decided to install a RAID 0 system in my PMG4; since as many mac users I wanted to improve performances of my PMG4 instead of buying a PMG5 (not enough space inside).
So I bought a RAID card and after installing it, performances were simply amazing for my MDD, as good if not better than my Office PMG5 Dual 2.0GHz.
The article is in French, but charts are easily understandable: http://www.jbmmv.com/technique/test-raid-g4/
If performance gain was between 180 to 250%, one should not forget that the internal PCI bus 33MHz will limit the maximal data speed transfer to 132MB/s (theoretical). He has used an Acard AEC6890M with 2 S-ATA ports.
But it is definitely a good way to boost performance of a PMG4 while waiting for the new Mac Pro which should arrive soon.
