yes as above, you can't use the nvidia automated driver search thingy, you have to select the correct driver yourself, download it and install.
this is actually easier now that it used to be, that new "one size fits all" verde driver or whatever it's called, seems to install on imac hardware at least without throwing errors like the previous 8800gs drivers used to anyway
good luck!
Update: Hm ok just read your latest post. What error message does it give exactly? Anyway, if you are 100% sure youre downloading the correct driver for your hardware, one workaround you can try (what I used to do before the verde drivers started installing properly on the 8800gs in my imac) is to go to laptopvideo2go.com and download the drivers from there. Those drivers are edited so that they will install on anything without throwing incompatible hardware-type errors. I used doxdrivers (outdated now) for ages on my imac without any problems.
If you're not sure what the heck laptopvideo2go is or why it is search in these forums as there were lots of posts about video card driver updating a while back.