Also, i think that the miner also uses one core, but i might be wrong since i only tried CUDA miner which uses 100% of GPU plus 100% of one of my 4 cores.
So yes, the electricity bill will hit you hard. Also, you need a new power supply, since usual power supply of usual Desktop PC will not be enough.
ArtForz just did the math for you https://bitcointalksearch.org/topic/at-what-point-are-you-making-a-profit-through-mining-2196
It is very profitable. Even if you take my 700W power unit consumption as basis.
The miner uses both GPU cores, each one crunching at +300mh/s
There is a problem /w ATI SDK 2.2 that prevents from running on both, but 2.1 works perfectly