The amount of dropped connections would probably depend more on how far away your mother's computer is from the wireless router/AP, how many walls the signal has to go through, type of material it has to go through, whether the prescence of 2.4ghz phones or other sources of interference exist in the house, the OS on her computer, etc.
I would suggest if the computer is far away or has to go through a lot of walls/interference that you get her a USB wireless adapter as opposed to PCI. (Also recommended if her computer is in an enclosed computer cubby in a desk) This way, she can get the best possible reception. It's amazing how much difference it makes by moving the adapter 8 inches to the left or right. (This is also dependent on your wireless source as well)
If her computer is relatively close then the PCI one would work fine. I wouldn't recommend Netgear b/c of the horrible reception I got with their USB adapter, but maybe their PCI adapter works better.
It really is heard to pick a particular brand b/c someone will have issues with each of the respective brands' equipment, so you might want to buy two of them and see which one works best