Summary of performance issues

Drive type | number of layers playback 30fps with dv-ntsc
-------------------------------------------------------------------
Internal -----| 2
Raptor ------| 3
Scsi -------- | 4



If you absolutely need to playback 4 layers on 2 outputs and you cant tolerate any slight slowdowns:

You need a single scsi 10000rpm atlas iV 140gb ( i tested with these )
You can use a single raptor - it will still work on 4 layers - the 4th layer occasionally will slowdown.
You must keep the operating system on a separate disc.

and you will need to use the dv-ntsc codec if you need/want 30fps on all layers.

If you are running at 25fps then dv-pal and dv-ntsc will both work.

Photojpeg does not decode fast enough - but will give you 3 layers at 25fps/30fps.

Other codecs will work without problems on 3, 2 and 1 layers
You exact performance will vary.

On my machine at home i can almost playback 3 uncompressed layers - rendered with the blackmagic YUV codec or 3 dvcpro 50 layers at the same time.

I have a dual g5 with a variety of drives.
My benchmark, sanity drive - is the 10000rpm Atlas IV 140GB.