Thanks pepedog for this info, I've been searching the web like mad!
For those stumbling about this issue: In addition to the steps described by paranoidandroid, you would have to do a
resize2fs on both the 2 and 5 partition...
Also, I've found that it is possible to simply change the partition size with
gparted as well (from a gparted live usb stick (
http://gparted.sourceforge.net/livecd.php)), rather than using fdisk, which I found easier to handle...