GTX, GT and GS are simply various models each with its own quirks. GTX is the most expensive and best and then there are the GT and GS. 7900 Specs link showing the differences between the 3 cards.

For overclocking check out sites like Overclockers.com that should have guides and warnings as understand that by running chips faster than they were intended you can shorten their life among other things.

Regards,
JB