I don't know Partition Magic, but we generally recommend GPartEd for partitioning - ext3 is a linux filesystem format (which Windows does not understand), and GPartEd is a linux tool, so it's the most reliable way. There's a step-by-step guide to doing this and installing a2sd+ in the Root Memory FAQ.
As for your data, it depends whether you are already using a2sd+ or not. If not, then you just copy the old card to the computer, partition the new card, copy data back onto it, and then follow the FAQ. If you are already using a2sd+ the simplest way would be to take a nandroid, copy everything off the old card (including the nandroid), partition the new card, copy everything back onto the new card (again, including the nandroid), boot into recovery and restore from the nandroid - that will copy the contents of your old card's ext partition onto the new one (whereas a PC cannot see the ext partition on a card, so can't copy directly).