@Melita
There's a free 3D program that mimics RL sculpting called Sculptris. If you want to spend some money, you could check out Zbrush, Mudbox or 3D Coat. I personally use Zbrush, and I have no complaints. However, I will say that it has a steep learning curve. You might want to give Sculptris a spin first to get your feet wet. Although, the tech in that program differs from some of the others.
As for Photoshop, you can paint directly on a 3D model in the "Extended" versions of the program. I know you can export to Collada .dae from Photoshop CS4 Extended for sure.