1.: DDR SDRAM is out already. It's usually referred to as PC1600 (200 MHz) and PC2100 (266 MHz).
2.: DDR SDRAM costs about as much as "normal" SDRAM ... a little more maybe, but not much.
3.: RDRAM - is it any good? (here's a big question). The anwer is - depends.
If you intend to use an AMD system, there's no point (not to mention possibility) to go RDRAM, as there's no chipset and the CPU's won't be able to use the bandwidth (if they were to go dual channel).
It's more expensive (at the moment, considerably so) than DDR or "normal" SDRAM. However, it is quite interesting for certain users. RDRAM is really only of any benefit on the P4 - like the Athlon, the P3 just can't make use of the available bandwidth effectively. On the "good" side of news for RDRAM is that it should be getting a lot cheaper still in the future - with new technologies, volume-sales & such helping (don't want to bore you too much with details).
If you're talking about DDR RDRAM - that will not come out for the near future (I can't say whether it ever will come out - don't deal to THAT level with memory). RDRAM will have a speed-bump soon-ish to 1066 MHz - I expect this to be publically available around the same time that Intel will do the speedbump for the P4 to move to 533 MHz FSB.
I hope that this answers your question. 😀
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