Did you configure it properly?

How could it run at just 800 MHz? Maybe the mainboard's clock wasn't set right, or the mulitplier was too low?

It's known that Via C3's can't compete with Intel or AMD counterparts, but no one expects them to. I know they have very limited cache (64 or 128 KB vs. 256 to 512 KB), which slows them down significantly. But as you said, they're "cool" and don't need much fanning.