There is still a limit. Just because you decide to multi-task and perform heavy gaming, it doesn't mean that you can up the wattage indefinitely. So if your GPU will at most pull in 200W, it will at most pull in 200W.
Depends. If multi-tasking, you mean a few word/excel/etc + browser, then it won't take up much power at all. Gaming alone will take more power than that. So it really depends what you do. The GPU will use up most of your system power. So anything GPU-intensive will rocket up the wattage.