Would there be any noticable difference between a 6600 and a 7300 gs overclocked by the manufacturer?

Answer 1

Not noticeable, you would only see a slight decrease when using the 7300GS. Yes I said decrease, the 6600 is better.

Answer 2

Dont know about those 2 specific cards. But I would think so.
See which has more pipes.
and then see how many locked pipes each card has.
Thats the total pipes because the locked ones can be unlocked using RivaTuner most likely.

You also want the to compare which one has more memory. The more the better. Also what type of memory GDDR is best.

And you would also want the one that has better graphic capabilities when they are not overclocked.