Win7 is pretty good at caching.
If you want better, try Fancycache with delayed writes enabled. However enabling delayed writes is dangerous, you can be completely screwed if you get a blue screen while a write is in RAM but hasn't been written to storage.
I used it for a while to try it out. Quite significantly faster for a HDD. Not so much for an SSD. THat was with 6GB RAM for the cache and with delayed writes enabled. With delayed writes disabled it was not too much different than the default windows caching that happens automatically.