Hey, check this out. 1 External Hard Drive for Your Mac and PC :: My First Mac - Help Buying and Getting Started with Your New Mac
I've copied and pasted it below but it doesn't seem to format correctly (unsurprisingly).
Leave NTFS if you are just using the drive to transfer files from the PC to the Mac. Easy - Nothing to do. You will not be able to transfer files from the Mac to the PC.
Reformat your NTFS drive to FAT32. Don’t forget to backup your files first! Easy You need to reformat the drive.
Requires no special software Limited to 4GB files or less.
Good if you will be sharing the drive equally between the Mac and PC. Some Mac applications my not run from the drive (not an issue if you are simply backing up though).
Reformat your drive to HFS+. You will need software such as MacDrive ($50) so that Windows will be able to read/write to the drive. Good if you think you will primarily use the drive on the Mac and only occasionally on the PC. Cost
Relatively more complicated to install/maintain.
Keep NTFS and install NTFS For Mac 6.0 ($30) so that the Mac will be able to read and write to the drive.