probability formula for exploding d6

I need to know the forumla for finding the probability of succes for rolling equal to or higher than a target number, on an exploding d6 (if you roll a 6 roll again and add to it).

i need 2 different instances of this.

1) you roll a d6 (exploding), add your skill, and compare that to the tn (chance of it being equal to or greater).

2) the chances of: d6(exploding) + stat(a) being equal to or greater than d6(exploding) + Stat(b). as well as the chance of them beating the other by 4 or more.

thanks in advance

oh and does anyone know of any programs good for these type of problems? preferably free ones.