I don't know about RDram being obsolete yet. Until we get some real-world experiences with GB users and their feedback in these and other hardware forums, it's all speculation on performance/price. When you start getting over 512mb of ram, price isn't a factor, everything is still expensive.
If you really want an I850E chipset and really want to go with RDram, my suggestion is to look for more info on the Asus P4T-533C and some genuine Samsung RDram (pc800 that is usable at 1066 speeds), or spend the extra money and go with 1066 RDram.