The system+bundled software should take about 5 GB. In my experience, 80 GB is enough for most people, but you know your needs better than I do. If you do a lot of video editing or something like that, you'll want more space. I think the only current Macs with 80 GB hard disks are laptops, which means replacing the hard disk is not super-simple. If you need to upgrade the drive, I'd take it in to someplace like CompUSA that offers a warranty. I think they'll do it for like $30.