If I were you I'd pick up a cheap CPU, maybe an e5200 or heck even a celeron will do the trick. Put it all at stock, and turn on all the CPU/mobo power savers like speedstep, etc. If you are *just* fileserving, you really don't need much CPU power at all. If you'll be transcoding, then maybe the quad will come in handy.
If you can put the other 4GB to use elsewhere, I'd do that. Just leave 4GB in the server. If you don't have a use for it anywhere else, then there's no harm in leaving it in the server.
If all you're machines are already gigabit ready, then you just need a gigabit switch to connect them all. If you do, then you can get pretty close to the same read/write speed as direct disk to disk. I think I peak around 80-90mb/s on sustained read/writes. Otherwise, on regular 10/100 networks I think you cap at around 30-40mb/s (not sure about that number tho).
For the OS... be prepared to experiment. Meaning, don;t install something and then move a gajillion files over, realize it doesn't do what you need and have to start al over again. I reinstalled I think a total of about 5 times before I got what I wanted.
I'd start by looking at FreeNAS. It's free, works well, and has good community support. Supports all the major RAID types. The only downfall I found was that it didn't support the newer hardware. So the P5Q might not float, but the P4 probably will.
Next I'd look at a variant of Linux. I use Ubuntu (the server edition), I've also heard CentOS is geared more towards server use. If you aren't already familiar with Linux there is a learning curve but its free, rock solid, and has all the features you could ever want.
Finally I'd take a good look at Windows Home Server. It costs some $$$ but is easy to setup, has a ton of features geared exactly towards a home user. Lot of extensions/plugins are available too.
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