Any modern computer IS a multitasking computer and is doing much more than you might expect while browsing the internet or even just sitting 'idle'. If you have a game which isn't CPU intensive (some games are more CPU intensive, some are more GPU intensive).
Easiest way to know is to run just Skype, check your CPU usage average for a minute or two. Then fire up just the game, and check the CPU usage for that. Add them together and if you're below 90%, you're good.