Right now the total network hash rate is about 25 TH/s.  When the ASIC miners hit the scene it will go up to 250 TH/s, 500 TH/s or maybe even 1,000 TH/s.  the difficulty will go higher and inversely your potential to earn will go down by the same factor as the difficulty goes up.

I explained it for another user here: https://bitcointalk.org/index.php?topic=139332.0

I have also covered some of this on my blog : See signature.

As long as you have the GPU use it for now.  Just know that in the near future things are going to change.  If you ask when, no one can tell you.  Although supposedly one of the companies started shipping last week.