I have had 4 burners going at once before with Nero 6 and posted my findings along time ago. It worked but the read ahead buffers couldnt stay full. It wasnt the hard drives or dvd drives or cpu. There were 4 separate hard drives on a ~3.6 ghz system.They all finished just alot of buffer errors.
Pioneer dvr-108,NEC 3540,BenQ 1640,LiteOn 1693S
Intel pentium D 930 oc to 4.2 ghz
2 gb ddr2 1066 mhz ram
sb audigy 2 zs platnum
hardware mpeg2 encoder card
Conscience is what hurts when everything else feels so good