If you are talking about the numbers reported for your card then yes you need to double them for the actual speed its running at (effective speed).
The indication is the DDR (Double Data Rate) GDDR3 is faster than GDDR2, GDDR4 is a bit better than 3 and GDDR5 works differantly and is, i think around twice as fast as GDDR3.
Pretty much yes, Its dependant on the aplication you are using to report the cards settings but as far as i know all the standard ones like the control pannel will report the lower number.
The sales guys like to print bigger numbers on the box and on the websites as it looks more impressive. It does depend on the type of memory used, GDDR2/3 and 4 are x 2. GDDR 5 is differant it needs the number to be multiplied by 4 to get the (effecctive) number.
Have a google about see what you can find if you are interested in knowing more and any questions or if you just have trouble finding some decent info then post back and i will dig out some links later for you.
yh thx man one more thing that is confusing me if my gt rins at 666mhz in games
so how come my old graph card is 400mhz (800mhz actual speed) the memory clock of the oldest is fastest so why does the gt 9400 run better WAY BETTER
Its all relative really. You didnt say what the older card was. The 9400 is a low end part and so its only natural that the clocks are a bit on teh low side.
If your older card was higher up say a midrange card for its day then i would expect the clocks to be around 800.
Yes it will be fine, the control panel is reporting the Actual rate and not the Effective rate is all. Im supplying a link that should answer all your questions and probably bring up some new ones if you read it all, its quite in depth. http://www.hardwaresecrets.com/article/167