I think if you look carefully, most of the those Power Requirements you've read indicate 20A on the 12V, not 20A on a single 12V rail. What I mean is that those requirements you've been reading are asking for a total of 20A of 12V power, whether it come from one rail, or multiple rails.
As long as those numbers you listed came directly from the label your PSU has sufficient power across the 12V rails to run the 7900GS. However, that can change over time as parts of the PSU degrade over time. I personally think from a numbers standpoint you're fine for now. It's the quality standpoint that you may run into issues over time.
I would use the Mercury for now but save up some cash for a good quality unit down the road as well as always be on the lookout for any good deals on the brands I listed in the post. A quality 500-600W PSU will last a long time and can be used in future builds as long the industry standards (currently ATX 2.0) stay the same and you don't get too crazy with SLI/Crossfire video card setups.
its more than enough you have dual rails. btw should got the 1950pro
1950Pro is priced 2K (approx 45$), more than a 7900GS pal. Radeon cards are priced so much high here, then I had headaches regarding my psu, wattage n amps, a x1950pro eats around 20W more than a 7900GS as measured from "extreme power calculator". Hence thot 7900GS wud be a safer bet :wink: