I have an evga 8800gts 512mb version and I noticed that the fan never runs fast on it. I noticed it first time when I say my 3dmark 06 scores are kinda poor. I fan always runs at idle speed (my case is open and laying on its side, so that I can look at the gpu fan). My system gets a 3dmark06 score of 13k with default settings.
As far as I know, this is pretty common on a lot of vid cards. On my card it would not increase past 70%, even when the card was in danger of overheating.
If you are concerned about your temps download rivatuner from guru3d.com so that you can monitor your temps. Rivatuner also has a great tool for adjusting the fan speed at different temps. Just follow the tutorial they have on the site. Good luck.
I believe that the 8800gts512 runs around 35%(+or- 5%) idle, and around %50 load(crysis). As said previously rivatune is a great utility to not only monitor the temps of your gpu, but also adjust the fan speed.