If you place your user data into a seperate partition, then 40GB should be plenty - unless you have more than 8GB of RAM where the pagefile and hiberfile already take a large chunk - each of those files takes the same amount as the size of the RAM.
If you don't want to bother with a seperate data partition (which is highly recommended), then add the amount of data you plan to store in Documents, Music, Pictures and Videos (which can take quite a bit of space).