I have a MSI GTS250 OC'd to 800mhz. Naturally I'd think the fan would rev up faster as i play games. At lower speeds, you simply cannot hear the fan, once the fan starts operating at 50-60% speed, you'll start to hear it. Depending on how efficient your heatsink is on the card itself, the card may not be hitting temps to require additional support from the fan at higher speeds.
One option is to use Rivatuner and manually set your fan speed, set it up high enough to where you can hear it, that way you can tell what it sounds like when its really workin' it.
Best of luck!