Hi JM,
My kids are 10 and 14, and we've never done a game system, because the amount of money it would cost would not be balanced by the amount of time I'd have allowed them to play (to me, you'd have to play at least an hour a day to justify the cost). Think of it as the same as television or computer time, how much screen time in total do you want to allow in a day? The 7 is old enough to decide how he wants to divide his time, if you allow an hour of screen time per day, he may want to spend all of it on the Wii and none watching television. I'd just suggest remembering that even active video games aren't a substitute for playing outside. I have a rule about good weather days, on a sunny spring/summer/early fall afternoon, no t.v. - I would do the same for video game systems. It's also fine to have the very occasional "unlimited" day. I find that if it's a snow day or one day over school break and I tell my son he can have unlimited t.v., he actually watches much less than I would have anticipated and it isn't on all day
Good luck