Hi  
    I newly have purchased a Gigabyte Geforce 8800GT 512mb   "GV-NX88T512HP" .....I run it under Windows XP proffessioal...my system spec are :
 
PSU 600Watt
Mobo Gigabyte 945P-S3 rev3.3
CPU C2D E6420(not overclocked)
RAM Patriot 2GB 667mhz
Harddisk Maxtor 160GB Diamondplus 10 7200rpm
Gigabyte 8800GT @ 700mhz GPU/1840mhz/shader 1717mhz --- memory 512mb (all spec from gigabyte box)
Nvidia driver forceware 169.21
 
Now my questions please  
1-Should I download the nvidia driver from the Gigabyte website in the support section ? or no need and do the job from nvidia website as they are all the same drivers ? I mean are the gigabyte drivers better and designed for their products or not ?  
2-What is the best programs or utilities to rely on messuring my new Geforce 8800GT temps and voltages ?  
3-Is the 9645 3DMark06 ver1.1.0 good or low for me ? using the demo version ..all the program setting to default (untouched)...
 
  Thanks if you can answer all my 3 questions...thanks

Well I get about 9850 under Vista64, 10,050 under Xp pro32.  Your score seems about right since your cpu is not overclocked.  You could up that score considerably by overclocking the cpu and/or the video card.
Download the Nvidia drivers from their website.  Theres probably gigabyte utilities on your driver cd that will monitor your gpu temps or you can use Ntune.