With unRAID, you have 2 options - You can write directly to the protected disks and I average 30 MB/s with WD Green drives (both parity and data drives are the slow green drives). Or, you can write to a Cache drive that is unprotected until a batch process at 3 AM that moves all the files to a...
Take a look at unRAID if you haven't already (http://lime-technology.com). It is the closest thing I have found to future proofing a file server. You can add a mix of drives and add larger ones as they become available. I have been running it for almost a year and have expanded the array...