4Gb will be fine. In fact for light usage I've found very littel difference between 2 and 4Gb with Windows 7 64 bit.
Edit: the lag almost certainly caused by the Atom processor, it's very, very slow.
Thanks. That's what I figured, that the CPU was the major issue. I've just always dealt with a CPU + dedicated GPU so wasn't sure if it would matter much. I know memory speed does. Just wanted an extra opinion before I pulled the trigger on the memory.
At least the motherboard does have four slots, so although it wouldn't be ideal, I could add in another two 2GB modules if she needs it in the future.