XDR Rdram looks intersting, but who knows if it will catch on outside of consoles. And if it does, assuming someone besides SiS designs a new rdram supporting chipset and it actually catches on, I'm betting the starting price for xdr rdram will be around, oh, say, one million dollars. Sounds about right for Rdram.
Extremetech has an interesting article up on it, give it a look if you're interested.
Look up Quad Band Memory, QDR, SRAM, eDRAM. Of course, on the nonvolatile side of things, there's the whole "RAM drive" idea, holographic memory, CompactFlash, SmartMedia, MMC, Memory Stick, SD, etc. etc.