I really wouldn't worry about RAID for your needs. RAID 1 can be slow and it's only a benefit if one of your drives fail; don't think about doing it as a backup. RAID 0 you are putting all your eggs in one basket. Any drives fail; your data is toast. RAID 5 is better for speed and redudancy, but I haven't seen it on too many consumer systems.
In my understanding water cooling is the choice to keep your system cool when you overclock your system. If you don't overclock, there's little need for one.
Most of the parts of the consumers systems are similar, but the support--in my mind--is lacking. You want to consider a bouqet vendor, Alienware you mentioned, but there are others, such as Voodoo or Falcon Northwest. They may provide better support than Dell, Gateway, or HP.*
*I am aware that Voodoo is owned by HP and Alienware is owned by Dell.