No matter what you will buy, 3, 40 or 1000GH/s device.
The difficulty will grow with the total hash rate.
As you see, even with current difficulty you'll earn only several percent of the price of your ASIC in the month.
And every month the difficulty will grow.
Your used GPU is still valuable for gamers, your used (1 year) supa-dupa-rig will be useless for everyone.
I consider entering ASIC mining is much more risky than it was with GPU.
By the way, if someone believes in ASIC by october, then buying BFL singles right now is insanity, payback time is at least one year with current difficulty.
And I forgot about diminishing miners revenue