That board is a LGA 775 socket on a P35 chipset and will tolerate virtually any Core 2 processor Intel ever shipped, which helps a bit since a desktop Pentium 4 reaching 45w total system power is...unlikely. However, even with a good Core 2 you're still using a desktop motherboard which is going to limit just how low you can get it. Basically the gold standard for Core 2 processors is the ultra-low-voltage mobile parts, which can reach as low as 10w (just for the CPU mind you, not the rest of the system), but reaching that level with a desktop board and processor is basically impossible because Core 2s are multiplier locked and due to the nature of how the clocks are generated, 1.6 GHz (standard idle clocks for most Intel desktop chips) is usually the underclocking limit.
There are a few tricks you can do to reduce power that most people would overlook: clock down the memory, loosen the timings, undervolt it if you can and only put in 1 stick instead of 2 (you don't need dual channel memory for a system that is only ever going to spit out huge media files on a network). Find a passively cooled video card and again with only 64bit memory (more bits requires more chips, which of course requires more power). Use BIOS to disable components on the motherboard that you don't need, probably doesn't save much power but can speed things up by saving the OS from having to load its driver.
Also "some crappy coolmax psu" (probably 500w) probably doesn't help that 45w target any, a crappy PSU probably draws 45w all on its own even if you only shorted the power-on pins without actually connecting any load to it. PSUs efficiency graphs generally look like a upside down bathtub but they are at their worst efficiency at low loadings; so asking for 45w from a 500w PSU is probably going to be near its worst efficiency.
Honestly; 45w with a "bunch of desktop parts sitting around" is probably not happening. The only way someone could really pull that off is to build a whole new system for it; but if its saving on the electric bill you are after, building a new system would probably cost more than it could save (granted, cutting 90w down to 45w in a 24/7 uptime system multiplied over a couple years would probably be talking...). Probably best just to aim for one of the slower and cooler Core 2 Duos (or a Wolfdale based Celeron), buy or borrow a kill-o-watt and see what the full torture test load power of the system is, then pitch the 500w clunker and buy some 80plus PSU that can deliver that amount of power (with some wiggle room to spare) and call it good enough.
Now; about the hard drives: 10k RPM RAID in a network file server is pointless in terms of performance and counterproductive in terms of heat/power. A single modern 5400 RPM hard drive will saturate the vast majority of gigabit ethernet implementations. Never mind that "fast drive" and "network drive" are all but mutually exclusive concepts. Here at my house I have a 4 TB RAID0 NAS box, which uses an ancient system on a chip that can only transfer 5 MB/sec, but it works perfectly for streaming every kind of media we have including bluray disk images to a media player (just loading 4 TB of files on to it is a ★■◆●
). So high performance drives are totally unnecessary in a network media server. Also considering low power is one of your goals, feeding and cooling a couple of 10k RPM (or even 7200 RPM) drives is going to burn through power without providing any benefits over slower and cooler drives.
If you want a fast drive in your desktop: get a SSD. I have an Intel 160 GB SSD in my system and the difference between it and a mechanical drive is astronomical. It is so much faster that it is basically impossible to describe, you just have to experience it to understand it and once you do you will never be able to go back to booting off a mechanical hard disk.
I am willing to take some parts from it if useful.
