I own a 9800 gx2 card from Leadtek and the last date for their driver i 2008/7/24 which I think is quite far back. Does it matter if I download the latest from Nvidia?
For a GeForce or Radeon, they usually update their drivers to increase compatibility and performance. After some time though the updates tend to do nothing significant, unless there was a severe problem.
Yes, download the latest from NVidia. The only time you wouldn't want to just use theirs is if the card is special/weird and needs the manufacturer driver, which is very rare and usually only necessary for, well, I can't really think of a good case. So just get the latest from NVidia.