150 GB is way too much for the OS drive, I'd go with a small drive for the OS and a bigger one for the scratch disk.
in my system I have a 36GB raptor for the O/S, 2X120 GB SATA drives in RAID 1 for projects and backups, 2X200 GB SATA drives in RAID 0 for scratch disks (very fast and not important data) and 2X300 for video storage. (I also have 2X300 RAID 0 external FW800 box)
You should also consider a video card like the matrox APVe with support for dual head display and HD output for monitoring (most graphic cards can't support both at the same time, so it's one screen and a monitor or two screens without the monitor)
in my system I have a 36GB raptor for the O/S, 2X120 GB SATA drives in RAID 1 for projects and backups, 2X200 GB SATA drives in RAID 0 for scratch disks (very fast and not important data) and 2X300 for video storage. (I also have 2X300 RAID 0 external FW800 box)
You should also consider a video card like the matrox APVe with support for dual head display and HD output for monitoring (most graphic cards can't support both at the same time, so it's one screen and a monitor or two screens without the monitor)