I'd stick to the HP xw 8200.
More speed (with less overheating issues as you said), slightly better graficcard and a better partitioning of the HDDs. You can put os, programs and virtual RAM on the 160 GB HDD. Set up one partition for each, especialy the vRam should have its own as near as possible to the center. That keeps the RAID for data only.
The 73 GB of the Sun W2100z seem a bit less to me for this solution and mixing SCSI and SATA can cause trouble either.