We have two boys (11 and 15) who are both very into their XBox 360 and computers...but we have them participate in regularly scheduled sports (wrestling and swimming), which keeps them very physically fit. They might complain a little bit when you first pull them away from the electronics, but they enjoy the activities and getting to be with other kids their age--face to face.
My suggestion would be to limit the time they are allowed to play games, be on the computer, etc. Also get involved with them in whatever type of outdoor activities they enjoy. From playing catch to just kicking around a soccer ball. Just get involved in your children's lives. It's a win, win situation.
If verbal encouragement doesn't motivate them within 15 minutes, I pull the plug out of the outlet or take the batteries. You'd be surprised how much activity they can whip up following you around the house crying and pleading.