5Ghs == 0.0289 BTC/week atm (@609.5M difficulty)
2013-12-02 difficulty will change, and you will earn less (~0.0222 BTC/week)

https://en.bitcoin.it/wiki/Difficulty

sort version:
difficulty changes every 2 week to compensate overal network hashrate changes
new/better miners join => increasing network hashrate => increasing difficulty => decreasing your mining device effinency

history for changes:
http://mining.thegenesisblock.com/

without energy cost you could run your miner forever
but there is a high chance your investment will never paid off, because difficulty increasing 20-40% / every 2 week

3 month later (6 changes) difficulty may be => 1,3^6 = 4,826809x higher!
so you will mine 4,826809x lesser with the same equipment


look around the forums
there are few incoming ASICs with brutal power, but making profit with them stil highly depend on release date/shipping time
1-2 month delay.. and they are worthless