You should be able to. Any ringtones I've ever downloaded just saved to my memory card.
But even if you don't actually download ringtones and move them to your memory card via the reader, you can make any MP3's (like actual songs) your ringtone as well
You can download to your pc and then move them over via the usb cable if you have one - that is how I do it. It is easier for me to pull them off zedge.com for instance and then open win explorer and just move them to my blackberry ringtone folder, done!
I have always used and prefer cards made by SanDisk. Never had an issue with them and mine came with an adapter for the card to fit into so it can go into the PC (maybe they all do).