What cards are you talking about when you say "350 dollar card with a core clock of 850 and a 500 dollar card of the same model with 950 clock everthing else staying the same."
The core clock of a card is a small percentage of what makes a card as fast as it is. There are so many other things that contribute to a cards performance, like number of ROP's, Stream Processor's, Memory bandwidth, Memory Type ETC.....