7900CS? I have never heard of that. I have a 7900GT and as far as I know there is the 7900GS, 7900GT and 7900GTX, in order of performance. I don't know what it stands for but it denotes where each card is in the family. As a basic rule for nvidia cards GS is always less than GT and GTX is better than GT and GTS.
It gets really confusing at times, that's why you have to examine the fine specs.
The difference between GS, GT, GTX etc would be price and hence performance due to the different bandwidths and clockspeeds.