Now that cpu's come in gigahertz speeds, the prices on 500-900 megahertz processors have dropped considerably. Any processor from 500 MHz on up will probably suit your needs. When it comes to disk space, PC's seem to come with enormous hard drives. Any more it's hard to find a new system with a hard drive smaller than 10 gig. Music files, movies and graphics are the things that take up the most space on a hard drive.
Games can also take up a fair amount of room.
Here again it's a good idea to pay for as much hard drive space as you can afford. If you're interested in graphics applications, digital photography or making your own movies, a graphics card with memory on the card is the way to go. This can speed up image processing and make movies and animation run smoother. When it comes to sound, you can get a lot out of a PC for a minimal price. There are a lot of inexpensive sound cards that can produce decent sound quality.