Why do they not build ram out of the same kind of memory as is used in the CPU cache?

[Nomaken]

I understand the memory they use for the fast cache on the CPU is expensive and large, but why is there not even an attempt at it? I can see that if they made the CPU 4 times as big, made almost entirely out of fast memory, that there'd be too much heat to dissipate, but why there are there not ram chips made out of that fast memory? Wouldn't there be drastic performance gains if there were 1 or 2 ram sticks with only megabytes of memory on them for the often referenced data? Couldn't OS's just be designed to know that a certain memory range is made of super fast memory and know to put the often used data in there?

 

[rgd1101]

as you stated it is expensive.

http://ark.intel.com/products/75122/
8 MB of smart cache