I looked into this issue a couple of years ago and found that there seems to be a slight difference in rendering times using a fast drive, but the most noticable differences were with Compressor.
I was using a dual 1.42GHz G4 at the time, and tested on an informal but continuous basis. The system had the Apps on the system drive and the media drive choices were FW 400 drives and an internal Ultra160 dual channel RAID.
I'd estimate that the render performance was maybe 5% faster, but when outputting a 1-hour MPEG-2 clip from Compressor (2-pass VBR, Best Quality) the task completed in 3.5 hours when destined for the FW400 drive; the same clip written to the RAID completed in 3 hours, about 15% faster. The differential was the same over numerous clips.
All SATA (or better yet, a SATA RAID) performance should be better than FW because the single drive channel rate for SATA is in the same ballpark as FW800 (but without the FW interface/arbitration) and it multiplies as drives are added to the array.
Although all the drives on a SATA card will be using the same (PCI) bus, each is on its own channel, so there is little contention. The same is true of the G5 motherboard SATA.