Software Raid Cache

I was wondering if it would be possible to set aside a large (larger) 
amount of system memory to act as cache for software raid.
I have a few xeon systems here with 4-8gb of memory in them and based on 
watching the performance of some mylex raid cards
jump up when we did a memory upgrade from 64mb -> 128mb I was wondering 
if the same would occur in linux.

If this is even something doable (tweakable) Im wondering what the perf 
gain from allocating say 2 or 4gb just to raid cache would be.

Comments ?