Butterfly labs sell some reasonable priced set ups..
50gh/s would probably be the minimum speed i would run or it wont be worth while.
Im new to this mining game aswell so please dont take what i say as gospel.
Im thinking of purchasing a 110gh/s setup and joining a pool.
Should hopefully make around the $100-$120usd a day in BTC using this set up..
This will obviously drop as the difficulty rises.
Because there has been a mass influx of miners the difficulty rates have risen
at an enormous rate making mining that much more difficult.
Setups that were reasonable 18months ago are now useless because of the rise
in difficulty.
Do as much reasearch as you can and educate yourself..