If you plan on mining Bitcoins in the future please take a look at this web site before investing any money into an expensive rig:

http://mining.thegenesisblock.com/

It takes into consideration ever-increasing difficulty in mining along with delivery time of your rig. Many deliveries are taking 2 months or more from the time of purchase so please put in a date some time in the future.

Of course, the difficulty will be skyrocketing by then but that's factored in. Even if you have a 2 terrahash/second rig the difficulty may make it impractical to mine sometime in the near future.