In the Pentium III days Rambus was already there but not really an option because it costed double or tripple the price of ordinary Sdram and there was no performance gain. (mostly there was even slower performance)

With the Pentium4 this has changed. Intel specifically designed the P4 to take advantage of some of the stuff rambus is capable of. Also considering that rambus prices have dropped immensely (up to the point where it is really competitive with DDR Dram) and you've got a great product.

Bottom line : if you stick to Intel chipsets (like I do), get rambus. No DDR ram will be faster. That is, if you want a really stable board.
If you're the risky kind, you can go for a VIA or SIS chipset. Those companies have succeeded in pushing DDR Dram beyond the performance level of Rambus, but (imho) at the price of less stability.
Also keep in mind that Rambus will bring out RIMM 4200 any day now, which uses 1066 MHz speed.