Linux, or any other OS does not know how the RAM works. As long as the memory controller is properly configured (e.g. refresh rates set for non-SRAM) then the OS does not care is it runs on plain dynamic memory (plain RAM), fast page mode RAM (FP RAM, from the C64-ish times), Extended data out mode RAM (EDO), synchronious RAM (SDRAM), any of the double data rate SDRAMS (DDR 1/2/3) whatever.
All of those support reading and writing from random places. All will work.
Now cache is a bit different. You do not have to write to it for the contents to change. That will get in the way. Still, it is somewhat usable. I know that coreboot uses the cache as a sort of memory during boot, before the memory controller is properly configured. (For the details, check out the videos from the coreboot talks during FOSDEM 2011).
So in theory yes, you could use it.
BUT: For practical tasks a system with 1 GB 'regular' 'medium speed' memory will perform a lot better than with only a few MB super fast memory. Which means you have three choices:
- Build things the normal 'cheap' way. If you need more speed add a few dozen extra computers (all with 'slow' memory)
- Or build a single computer with a dozen times the price and significantly less then a dozen times the performance.
Except in very rare cases the last is not sensible.