Stay away from RDRAM!

Don't go near it, consider it hazerdous material.
Actually that's my personal opinion, but I don't see RDRAM becoming the baseline for the Ram industry.
The only problem is, the Pentium 4 really can't function its best using anything other than RDRAM (which peeves me, but the backdoor dealings of Rambus Inc. and Intel are not necessary for this). If you are going with a pre-made computer (and are purchasing it in the near future) and it has Pentium 4 in it, go with the RDRAM.
As a side note, if you do go the route of building it yourself, a new Dual DDR platform is coming out for the Pentium 4 with the Granite bay chipset. For the drop in price (sith only a slight drop in speed) I think it would be worth the wait for it. Plus most of the Mobo's using the chipset are decked out with all the features (AGP 8X, USB 2.0, Gigabit LAN, 5-6 pci, 2 serial, 1 parallel, 2 IDE RAID with serial ATA, Realtek 6 channel surround sound, and firewire)