Welcome to the forums.
Just a note: please only ask a question once. Asking a question multiple times causes confusion.
If you have an Optiarc AD-5260S (essentially a AD-7260S), you probably don't need a 7280S. The iHAS124 might be a good additional drive. But The AD-5260S is good on its own.
And you replace a drive when it starts failing & you cannot fix it. As long as it works, keep using it.