Hard Disk size is measured in gigabytes Gb (billions of bytes) andlike everything else in the PC world continues to increase in size. 200Gbdrives are readily available.
There are a couple of factors that govern hard drive performance access time and data transfer rate.
Access time can best be explained by thinking of a record turntable. When youplay a record on one of these, you place the tone arm down on the track youwant by lifting the arm, moving it over the track, and then gently setting itdown, right?
On a hard drive there are magnetic platters that spin around at 5400 or 7200RPM. Over each platter is a read/write head that can move back and forth acrossthe platter like the tone arm of the turntable. The difference is theread/write head never touches the surface. When they do it is not by design andis called a head crash which ruins the hard drive!
When your PC wants to get a file from your hard drive, the read/write head hasto move to the track that contains the file. The time it takes to get fromwhere it is to the track is called accesstime.
Once it is over the track it begins reading data from the track. The rate atwhich it transfers data from the track to the PC is called the data transfer rate.
Access time is dependent on the RPM of the hard drive and the data transferrate is dependent on the electronics that actually read the data from theplatter.
Besides those two factors, fragmentation comes into play. Why, because a filethat is in fragments means the read/write head is constantly jumping fromfragment to fragment, which takes time, rather than coming down and reading thefile from one big continuous section!
For general-purpose computing and storage of a moderate number of filesincluding digital audio and photos, a 40Gb hard drive should be plenty. Don'tworry about the drive RPM too much, because when you first access a file it isimmediately copied to memory. Subsequent accesses to that file are from memoryand the caches that support it, not the hard drive. Your changes won't be savedon the hard drive until a save is performed.
This why saving your work frequently is very important. Until you do, all yourchanges/updates are only in memory, not on the hard drive. I save changes oftenwith a quick 'alt-F-S" which forces a write back to the hard drive.
For reference, 200 MP3 files take up about 1Gb of space. 1500 digital picturesin low compression jpg format also take up about 1Gb.
If you want to store 'lots' of digital files like audio, video and photos, thena 80Gb-120Gb would be a better choice.
In either case, look for a drive that will support ATA-133 for a high datatransfer rate, instead of the slower ATA-100 or ATA-66. This will give youbetter performance. Most new PCs have disk drive controllers that can supportATA-133 disk drives.