That's WAY too much for 640mb 8800 GTSs. For $100 you can get an HD4850 new that is a much better card;http://www.newegg.com/Product/Product.aspx?Item=N82E16814102824
It would be a nice upgrade as your current card is really rather out of it's league for gaming at your resolution.
For something better that will perform well at that resolution for a while and is DX11 compatible this HD5770 would be a good choice for $153 if you can swing it;http://www.newegg.com/Product/Product.aspx?Item=N82E16814161317
You can use your old card in your sister's machine.
For video editing(and gaming for that matter) another 2 gigs of ram would be wise. Overclocking that processor would also be a good idea if your motherboard allows for it.