Advanced Raid 5 question

Im done massive amounts of research and I can't find anyone who is dealing with a problem similiar to mine.

I have (2) 2TB RAID 5 arrays on one computer (6hdds). I have extremely abysmmal write performance.
Here is the tricky part: I am writing files that are over 100GB a piece. Ive got the 3WARE raid controller maxed at 256Kb stripe size which has increased the write performance considerably from what it was when it was set at 16Kb. However, Ive been reading about the relationship between stripe size and cluster size and that a properly tuned system can increase my write performance dramatically.

I am a noob. I was tasked with this through my job and have taught myself quite abit over the last month but now Im getting to things I know absolutely nothing about.

Anyone have any suggestions?

This is where my noobishness comes in to play. Im not sure what benchmark software to use but I can tell you this, it took 20 minutes to copy a 4gig file from raid array to another in the same computer

Some hardware specs for the computer: Dual Xeon Quad(not sure on speed) 4Gb ram,
3ware 9550SXU Raid Controller, XP pro. This is all I can remember off the top of my head. Any recommendations for what software to use?

As for screenshots, well the computer is not connected to the internet (and will never be)

Wow thanks for the great replies. I will update this post tomorrow with as much of the requested info as possible.

Im almost gonna have to write the info down because thumb drives arent allowed to be used on these computers. I could go to jail if I attempted it.

I downloaded HD tune pro and will install that as well as check driver/firmware tomorrow. Once again thanks for the info and sorry I haven't been more specific thus far.