It depends on what kind and how much data you need to save. No matter how big your drive, it doesn't take long to fill it up with useless crap. I have two PCs, each with 2 x 80GB drives. Each drive has 3 partitions, C, D, E and F, G, H. I keep different types of data in each partition. H, for example, is solely for data backups, before I burn a DVD. I've never filled up any partition. I don't download useless crap. Anything that doesn't need to be on a hard drive gets deleted, or burned to a CD and then deleted. My hard drives are only for items that need quick access.  

1/2 TB (500Gb) seems to be around the average now, excluding any external drives that loads of people have now. I've got a ton of them personally, ranging from just 320GB to 2TB! I mean, here is a super cheapo computer and its got 500Gb: http://www.ebuyer.com/product/164667 Go up the tree just a little and they've got 640 GB http://www.ebuyer.com/product/159381 Once you start hitting the mid-range, 1 TB is common http://www.ebuyer.com/product/160514 Drives are so cheap now - around £40 for 750Gb... its less than I'd spend in the pub on a Friday night! Myself? I have RAID in every computer, so I've got at least 2 x 500 GB in each.

It depends on the system and what you do with it. My tower has three old drives totalling 170GB and I run into space problems with it (I need to upgrade) because there are a lot of games and a lot of videos on there. My netbook has a single 160GB drive with my entire music collection, a few videos, some games, and over 100GB of free space. Proof positive that it really depends.

As jerv says, it really depends. The REAL questions are what do you do with your computer and what do you need to have readily accessable on your hard drive? There are two primary files which eat up disk space: video files, which the vast majority of home computers would be concerned with. And files for certain programs like CAD (Computer Aided Design). CAD files can be truely huge as well. So, if you are the average home computer user, then 320 GB will be more than enough working space for you. This is because virtually every working program you need will only take up a tiny fraction of your total hard drive. And the rest is likely to be files that, once you reach your limit, you should probably move to a backup hard drive anyway because you likely won't have a need to access them frequently. An example: My wife's desktop has a 320GB HD. She records a lot of TV shows on her computer. Once the HD is full, I copy her TV shows and movies onto an external drive to free up space on her HD again. She will not be viewing all these shows all the time...only a select few at any given time. Then they are essentially recordings she just wants to keep for some vague time in the future.