Depends on how you compare them.......
RDRAM is no doubt faster, than DDR, but the gap in performance in no way warrants the high price of RDRAM (come to think of it, DDR prices have jumped 100% from when I bought mine!)
If you overclock, that increases the performance of your memory, so in a synthetic benchmark like Sandra, the scores edge OC'd DDR in front of stock RDRAM (see below)
As a comparison, here's some hard figures from the UK.....
512Meg (2*256)of PC-800 RDRAM is £162.15
512Meg of Crucial PC2100 DDR is £131.60 (It's getting dearer! I got a 256Meg stick of Crucial PC2100 in December for £35.....)