I registered just to post this...
I'm using a nForce 980a SLI motherboard, with 3x 1.5TB Caviar Green drives in RAID 5. I use it mainly for large files like movies, and copying back my movies to the array was VERY slow (~15MB/s). I thought I just had to deal with it since RAID 5 had a slow write (or so I read), especially with these Green drives which are not meant for performance... until I did a desperate search about RAID 5 on nForce and found this topic.
My partition was properly aligned already (Win7 did this for me), but all the rest, I left to default. My stripe size was set to 64, so I recreated the array with 32. Cluster size was 512 bytes, changed it to 64k.
I just copied a large file at 90-110 MB/s!!! This is awesome!
Edit: After just a few GB's of write, it dropped to 75MB/s, but that's still way better than 15MB/s