To reduce the size of system restore...
Right click on computer, click on properties, click on system restore tab, then use the slider bar to reduce the space taken up by system restore or you can check the box to turn system restore off all together.
Your downloads will still be there. As far as what net frameworks you need, I have no idea. As far as I know, There are only a few programs that actually need it and one is CCC but since you have nvidia, not sure if they require it or not.
I think once you shut off system restore, you'll be fine for awhile.