When you separate the boot disk, PS scratch disk, and virtual memory disk, it isn't really sufficient to put them on different partitions of the same drive. To get the most benefit, you need to be able to access these drives simultaneously. This means that they need to be completely separate disks (actual hard drives) on separate SATA channels (on older systems that used PATA, you also needed to put them on separate cables).
So, your proposed layout will have minimum, or no, added benefit.