It depends on the platform you're going to be using it on. On an i820 Pentium III system, RDRAM will be the fastest thing you can buy, but the price is simply way too steep to justify buying it. The performance benefit offered by RDRAM pales in comparison to its extremely high price. If I were you, I'd go for a 440BX setup with 256 MB of PC100.
I did a quick search and found this article
, which reinforces what I said with some benchmarks. It's your choice, but I think RDRAM is a waste of money.