The problem with onsite calculator is that you don't know the details and I am not sure if it takes into account electricity costs and rise in mining difficulty over time.
For instance I can assume that 7950 card makes 500 MH/s, that gives me 48.98$ per month with rate below 100$ per Bitcoin (following calculator).
If no costs are taken under consideration I can get my money back for the card in 7-8 months.

Actually there is thread about mining or investing instead:
https://bitcointalk.org/index.php?topic=56475.0
However it is complex for me. Do you have more simple excel or source for making calculations?

It is nice read as I can consider same setup  and now I know optimal PSU power from the story.
Thanks a lot.

@mynameismud
 Thanks for the link. It is useful although they do not ship international, so I need to look for local supplier.
In this configuration they use 650W PSU , but I thought about 550W for two cards HD7950.
Maybe it is worth to add extra Watts...
Also I read that Linux configuration is more stable however it is not so flexible for overclocking.

As I do not want to create new topic I want to ask is it still good idea to do GPU mining.
I want to buy new gaming/mining computer and I think about HD7950. Maybe then I could upgrade it to 2 HD7950.
How much Watt consumption should I assume for two graphics cards with AMD Phenom II processor?
I would like to mine when I do not use this computer.
You mention about mining LTC. Does this makes more sense?