ok think of it like this. A 19th lvl rogue, 1st lvl wizard...
Obviously the rogue is going to be VERY versed in sneak attacks, and he will know the MOST critical and vital area's on the body in order to do an attack.
now, the rogue has two options, he can sneak attack with some powerful dagger i dunno, dagger +5 with some flame effect doing 1d4+5, +1d6, + snmeak attack fire dmg...
or he can ray of frost 1d3 _ sneak attack. That right there proves its not overpowered. Yeah he can cast his little 1d3 dmg spell 6 times.
or he can attaack with his dagger 1000 times a day doing his 1d6+5+1d6+sneak attack. hmmmmm.......
Now, for ur visualization....
19th rogue, 1st wizard....
he is sneaking through an alleyway, in the darkness. Comes upon the evil warrior (with fullplate) tailing him for years...
(not much thoguht into this, I know) at a distance of , oh lets say 20 feet, he zero's in on the location. He quickly studies the oponent looking for that vital area (which he has had 20 levels to determine vital area's). He notices a slight slot between the helmet and his plate armor. He begins casting his ray of frost, Using perfect aiming (many dex bonus's plus a large base attack bonus) he shoots forth his ray of destructing, connecting at the very back of his neck.. The freezing ray strikes the backbone sending a paralyzing chill up and down his spinal cord which archs into his brain freezing his insides and leaving him stunned with pain.
Theres ur sneak attack 1d3 +9d6 dmg.