If you're going to be doing video work, then the P4 + RDRAM rocks. Samsung is generally
considered to be the best quality RDRAM, so that will give you your best chance to overclock.
The motherboard that you choose needs to have clock generators that support a 533Mhz FSB,
more info on that can be found
here.
I spent quite a bit of time researching this exact question for a friend that wanted a editing station.
My recommendation to him was an ASUS P4T-e and Samsung RIMMS with a P4 1.6a, running at
133 FSB for a final clockspeed of 2.1Ghz. Unfortunately, he decided not to build it, so I can't tell you
how it went. According to what I've read, that's the way to go. But, if you can wait a month or two,
the official 133 FSB parts should be out and then you wouldn't have to worry about overclocking.