Bitcoin mining may reduce the lifespan of my video card? I'm not using a mining-dedicated videocard, i'm using my only video card (Radeon 6850) and i'm getting around 0.11 btc/24h.

My concern is about reducing the lifespan of (or even burning) my video card with this.
Heavy usage will certainly decrease the lifetime, but I think it's not incredibly much as long as it's not overclocked. You will probably even buy a new card (because the current becomes too outdated/slow) before the current one breaks down.

Keep an eye on the temperature for a few hours (or as long as it's not stable/maxed out), it shouldn't exceed about 75-80 °C. Otherwise you may want to invest on cooling, regardless of bitcoin mining (games also make the card warm). If it's at an okay temperature, you should be good.