Its nothing tooo special in serious sam, It shows all the monster info, wepon info and stuff like that on the right monitor, and game on the left, nothing too special, but it was cool none the less. Never played ss on a single monitor system(played it at my friends house) but i assume you'd have to switch from game back to info, as apposed to just glancing over to the second monitor.
My favorite part was two players on two monitors, oh jebus, that was fun!!!!!