Well, unless, I am misunderstanding what running in parallel is, the only way you will have redundancy is with a RAID1 in that configuration. Which will mean either a hardware or software RAID controller which I doubt you will be able to set up on a Dell Optiplex.
Even if you did nothing more than purchase the 699 dollar server with the OS, you will be far ahead of what you were thinking about.
Now, if you wanted that RAID, you could add another 160GB drive ($110) and a RAID1 controller ($220), and you are still way ahead.
The number of features is hard to describe. I would recommend a few things.
OK, while typing this (and I must be seeing things) but the Dell price just dropped to $629.00! Personally, I would chat with Dell (you can get right through) and ask about the deal and what support would come with it as far as setup. If you purchase it from a Microsoft Partner (which I am, but I don't know how to sell it and you wouldn't want my support), they give full support.
