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).
Care to expand on the for the most part part?
![Smiley](https://bitcointalk.org/Smileys/default/smiley.gif)
From time to time you got a coin that allows you to set and forget, albeit, they are usually not extremely profitable, but they do earn their bread (or rather, electricity).
But since there are so many more miners today, this even itself up rather quickly, so 3-7 days of mining the same coin is what it's at usually.