In preparation for PSU reviews soon to come, I did some prelim testing of a power draw meter I devised. This gizmo allows the AC current drawn by a system to be measured, from which the total power (watts) can be easily calculated.
What sort of "gizmo" are you using? I find that your common garden variety multimeter on the 10 or 20a setting works just fine for measuring current with little if any chance of mistake and no calibration required. Also lets you measure the voltage instead of assuming it is 110 or some other value.
Just under 100w for the system you described is not out of the question, especially with your description of the synthetic load you provided "100% CPU usage, full video, all drives working". A program like prime95 or memcheck86 will do a better job of loading the processor to near its capacity. Multitasking that processor to try to keep the drives busy as well as play a video may show 100% cpu utilization (actually, 0 processor available, a subtle but significant difference), your processor is actually spending a lot of time waiting for main memory and not generating much heat. That and the whatever program you are using to keep the drives in constantly seeking end-to-end is not going to do much but wait for the PCI device.
While knowing that your system is generating 100-150 watts of heat in operation is helpful in designing a quiet cooling system, it doesn't do a whole lot in helping you in power supply selection. For that you need to measure the current on each of the rails and select a supply with adequate capacity on each. A 300 watt supply rated at 10 amps of 5v and 20 amps of 12v won't run your system very well since abit derives vcore for the processor from +5 on that motherboard whereas a 200 watt supply rated at 20 amps of 5v and only 15 amps of 12v will run it just fine. That same 300 watt supply with an over abundance of 12v capacity would run a different motherboard that derives vcore from 12v just fine.
good luck on your project.