It looks like you have received lots of advice! I'll add mine,just because it have been a life saver for me. I have four children, the oldest being 8 and 7. They both love the computer and would play for hours on end. I finally came up with a plan that everybody seems to enjoy.
I made up some simple "computer bucks" that they can earn by either reading, or practing their musical instrament (but you could include homework, or whatever activity that you would like to encourage). They earn one "buck", or one minute on the computer for each minute they read or practice. Anyway, my kids have really enjoyed this system. They read like mad, trying to earn and save up these computer bucks. And then, I let them redeem them as they choose (a little at a time, or in a big lump sum). This way, they feel like they have control, and I am not always having to say "no", and I am getting that moderation that I was looking for. By the way, I didn't make any $1's, just $20's. That way, the time is easier to keep track of, and it also pushes them to read a little bit longer or practice the piano/guitar more to earn that next $20 computer buck.
With T.V....I don't know, sometimes kids just need a little of that unwind time, so I haven't really limited this, or included it in the "computer bucks", but I just try to use my own discretion, and when I say it's enough, they just know that it is enough, and I get them doing something else.
I do have to agree with the last response that our kids live in a different world now, and their success in school and work will depend on their computer ability, and (as my husband puts it) their eye-hand coordination that comes from play-stations, etc.. I didn't buy this line from my husband at first, but then I got thinking about a lot of the jobs out there. So many of them require the use of these devices and increasingly, the ability to use a remote. The important thing I feel, is moderation, and that they still must develope their social/communication skills.
Another sidenote that may be different than some of the other responses, is that I don't allow my kids to play the computer on Sundays, it being the "Sabbath". On Sundays I do ask them to find other activities to do. This is where their creative play comes out. This may also be a good day to designate as a time when you spend time together developing other hobbies or interests like: baking, scrapbooking, gardening, writing in a journal, building with Legos, seeing what you can make with play-dough, writing letters to relatives, going for a drive to the mountains, or whatever activity that you can enjoy doing together. I've just layed that law down and they haven't seemed to mind it one bit. It actually served as a nice brake from the rest of the crazy work/school week.
And incase if you are wondering, "Zoo Tycoon" is one of their favorite games to play on the computer. It's a good game that can be picked up at Target or Walmart, and it's not super expensive. They get to design, create and run a zoo all within a budget. I've played it, it's fun.