[Animal]

If I was building a P4 today, I'd grab the Intel D875PBZ motherboard. It supports the latest 800MHZ FSB P4's, Dual Channel DDR400, Serial ATA RAID, 6 USB 2.0 ports, and all kinds of other goodies. At $175US, it's a little out of your price range though, but the more you can afford to spend now, the less you have to spend later to upgrade it.

A few notes:

RDRAM seems to be a dead horse. Look for a board that supports Dual Channel DDR. Not just DDR, it has to be Dual Channel for best performance.
The Intel 850E chipset based boards are pretty decent, but they only hit a 533MHZ FSB and support RDRAM only. This chipset is at the end of its road.
6 PCI slots seems a little bit much? I'm not sure what you could possibly need 6 PCI slots for, but you'll find most boards have about 5.

I've said it once, and I'll say it again. If your budget is $120 for your mobo, spend $150 instead. Cut corners somewhere else, but the motherboard isn't the place to do it.