Intel RST is software raid, it doesn't have a dedicated CACHE to write disk (like a dedicated Raid hardware).
Now, you need to enable RST write cahce. But...
But enabling RST raid cahe (from Intel RST software) is useful if you don't enable Windows disk write cahe also.
So, first you MUST ENABLE DISK WRTIE CACHE in Windows OS (server 2008) to have reasonable performance in Intel RST Raid 5.
To enable disk wite cache folow this guide:
Or.... buy a dedicated raid hardware!!