If you're on Leopard, then I do not believe you can get iCloud. I have both a PowerPC Mac Pro and an Intel MacBook Pro. For my PowerPC Mac the highest OS I can go is Leopard, and I do not see any option for iCloud for it. If there is, I would really like to use it, but I've searched and haven't found anything yet. MobileMe will be discontinued sometime in June or July 2012, hence, all PowerPC Macs will be without MobileMe or iCloud. They're being left behind.
To answer your other question, Apple is offering upgrades to iCloud, and you will not lose any of your data. You can keep your @me.com email address (iCloud uses the same @me.com address anyway, even if you're new). I have even been able to keep my @mac.com address from several years ago, back when it was dot-Mac (.mac). I believe you can upgrade through the iPhone or on any web browser (probably most convenient anyway).