Extra memory is needed to run multiple applications simultaneously. For example, you might want to have a word processing document open at the same time as a spreadsheet, and check your e-mail without exiting either. Buy at least a six GB hard drive. Eight GB is better and ten better still, but the price goes up significantly after ten. Include a CD or DVD drive.
Most new software requires a CD or DVD player to install.
You might also consider a zip drive or other removable media for making backups of critical data. The computer should have either a built in Ethernet card, or a free expansion slot for an Ethernet card. Don't skimp on a monitor. You spend hours at a time looking at the monitor. A monitor that isn't crisp and bright is very tiring to the eyes.