When I try to reinstall Mac OS (after wiping the partition with ubuntu) all i get is a blinking folder
The Ubuntu installer probably reformatted the whole drive using ext2 or ext3, whereas current versions of Mac OS X will only install on HFS+. To fix this, boot up with the Mac OS X install CD, then run Disk Utility from the Utilities menu. Disk Utility will let you check to make sure the drive has at least one HFS+ volume on it, and if it doesn't, create one.