Corsair, according to my opinion, belongs to one of the best PSU manufactures in the world.
The quality control is good. I have used more than 20 PSUs from Corsair in about 10 years and untill now I have never had any kind of problem. (I sell/build new rigs for my secondary living)
Experience do not lie.
Corsair always underates their PSU power ratings, which a good thing.
I think most of us in this forum will agree on that.
If something is wrong or sucks with your PSU...there is a possibilities that your whole system goes down with it...
It will be already too late..
I just want to second the Corsair PSU recommendation. They are very reliable units with excellent efficiency (especially the HX series, but you can't go wrong with any of their units).
You're saying that if your power supply sucks, you can just get a new one later...but what if your unreliable power supply takes other components out with it? Or decreases the lifespan and stability of your rig due to unstable power?
Just get the Corsair or Antec.
Also, why 12 GB of RAM? Is this computer primarily for gaming? 12 might be a waste of money depending on your useage.
Also, +1 for Spinpoint F3 drives.
If you have the $3300 budget, I'd like to see an Intel X25 SSD in there. You can save money by not buying the other 6GB of RAM and getting an SSD instead, which will result in MUCH more noticeable everyday-useage speed gains.