http://store.apple.com/us/product/MACOS_XSNGL it only costs 29 dollars to upgrade 10.5 to 10.6
But you do need to have:
Mac computer with an Intel processor
1GB of memory
5GB of available disk space
DVD drive for installation
Apple Menu, about this mac
the window that pops up will include how much ram you have, as well as processor type.