Price and speed points change with time. A faster machine can deal with sound and graphics better. Buy at least 64 MB of memory. 128 MB is better. More than 128 is nice to have, but not necessary for most of today's software, and can be added later if needed.
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.