I have 4 WD 80 GIG drives on A Asus A8N-SLI set up as a Mirror \ Stripe , I am doing a clean install of windows XP. Out of the 80 GIG's , what would be the correct partition size for NTFS file system?

And is it beneficial for me to partition the OS from data storage on this stripe mirror,and what are the benefits of such a set up?

The entire array needs to be formatted with NTFS, but I don't think that is what you meant to ask. I think you meant to ask, how big does my operating system partition need to be? The answer depends on how disciplined you are about making sure your programs and data ALWAYS end up in other partitions.

Most here would advise that you have separate partitions for the OS, for your programs, and for your data. If you are religious about keeping all data and programs out of your OS partition, you can get by with 8-10 Gig for the OS. I generally set up the OS partition on mine for 15 gigs, knowing that there will be some programs that write to the all users local settings directory and I don't generally take the time to redirect them, other than e-mail storage and browser temp files.

The benefits of separating your programs and data from the OS are in recovery after a virus attack. Most viruses stop their destruction with the OS partition. That means anything written to data/program partitions is salvagable.