You might be able to underclock it with rivatuner and create profiles to clock back to stock or oc when certain apps (ie. games) are launched. Personally that is a pain to me.
Windows has a marvalous feature called "standby" if you are worried about power usage just set the power button or if your keyboard has a sleep button to suspend when you push it. I do this at night because I am not using it for hours on end. I leave it on during the day if im downloading stuff. That will save you some power. Other than that I believe nvidia is coming out with ahybrid sli structure that will use on-board graphics for 2d and when it needs it power on the video cards. Its not here yet but its a good concept.
If she is just surfing the net it is still operating at basically idle power use. When you game is when it is going to crank up the power consumption. I don't think the benefits of a 2nd system will outweigh the cost in power. Plus then you have 2 systems to worry about. Personally I have a laptop that I use to sit in the living room and roam free. It's my "craptop" but all I use it for is surfing and a few other things.
I was thinking to do something similar to my 8800gt, but I thought of relating the underclocked and overclocked profiles of rivatuner to the GPU temp.
Let me explain, when OCed my GPU Idles at 43ºC, so If rivatuner detects the temp to be around 45ºC then the underclocked profile should kick in, then, (here starts some speculation because I haven't tested yet) lets say when underclocked at full load the card goes to 50ºC at around 48ºC the OCed profile should kick in.
In theory it sounds as a good idea to me at least, but I'm not very knowledgeable of rivatuner and I haven't got the time to fiddle with it because school started a week ago
If I get some free time and manage to pull something off I'll come back to share.
first of all sorry for my english.... there is an easier way to this....use rivatuner that thing is great... u just need to create 2 profiles.... one where the frequences are normal... and other with them underclocked.....but the trick is the way it chooses from wath profile should it take, if hardware acceleration is detected..(it means if a game or anything needs some real power)... then the normal profile is on.... but if u are using it for internet and somethings like that....the underclocked profile takes over.... and thats it.. really easy....