You asked if one GPU mining is feasible.
The problem isn't lying with 1 or 10 GPUs, but rather that generally speaking, GPU mining isn't profitable anymore unless you're really vigilant as Amph said.
Setting and forgetting doesn't cut it anymore (for the most part).
Okay, got it. Thanks for a straight answer. Now extension, was it ever?
Oh yes, there were a few different times where gpu mining was VERY profitable, but I'm afraid those times are gone, I for one, am selling all of my gpus, about 17 of them, so that I can buy asics. They are pretty much the only way to mine anymore, and even they aren't that profitable, but I just like mining, so I will continue to mine. There were times when I made 75 a day with only 3 280x's, then times where those same 280x's made me 1BTC in a day when MAX came out. Good times, but those times are long gone. Sucks for new miners, but it is what it is.