Photoshop CS6 requires a OpenGL 2.0 capable system. The Intel Pentium G3420’s integrated graphics is an Intel HD, which is supposed to be OpenGL 4.3 capable, so you should be covered. Other artists recommend using a dedicated graphics card for PS CS6, but that’s only important if you need CS6 features like rotating the canvas and using the photoshop filters. If you’re just doing digital painting, the integrated graphics will suffice until you can upgrade.
Since you’re planning on using Photoshop, the more important part to upgrade is the RAM. 8GB is a healthy amount, but if you’re painting a large picture (3000pixels x 3000pixels) with lots of layers, that will eat up a lot of RAM. Photoshop can deal with this by swapping memory to the hard drive, which slows Photoshop down. Some people hate when that happens, so they load up their computers with more RAM so all the open windows in Photoshop can fit in memory without dealing with swap.