Both Corel painter and Photoshop are resource heavy and an 3rd gen i7 gives you your piece of mind knowing tht you have a high end processor.
When it comes to software, in the adobe range the standard painting program is Adobe Photoshop as it is a raster based editing program. Corel painter is also a great piece of software.
Both have their perks and downfalls but either one of them will work well for you.
If you are going to be painting digitally then you should invest in a graphics tablet. The leading brand is Wacom however after many years of using a wacom i recently switched to a monoproce 12"x9" tablet and it works just as well as a 3rd gen intuos if not better. [link]
However if you want to stick to bamboo then do a little searching around to find a lwoer end one (such as a bamboo) There are also the genius range of tablets.
Not really for you, no. The Quadro's usually are optimized for GPU computing which is what you'll find using Phothoshop and the like. The filters and whatnots can be done on the GPU to speed stuff up.... but it doesn't come up as often while painting.
But the 660M is much faster than the Quadro so it'll slightly outperform the Quadro. It depends what exactly you test it with in your case it makes almost no difference. I flipped a coin.
The y580 linked by PR-Imagery is what you should get.