We need to know the capabilities of the machine
http://virt-tools.org/learning/check-hardware-virt/ If a computer has virtualization, it could be good for a power user.
This single instruction multiple data set is required to be able to run Google Chrome, and Flash. If a computer doesn't support this, it is only suitable as a toy.
While this isn't really that important for most people, if the computer is 64bit it is more likely to work in the long run. Computers that are only 32bit are ok for people who will be able to get another computer in the near future. 64bit capable computers should go to people can't handle switching computers in a few years. PowerPC apples are basically toys.
If a computer only supports DDR, SDRAM or RDRAM, it is probably only useful as a toy. Something to play with.
DDR3 is as of 2014-11-01 is the cheapest type to get in a reasonable capacity. DDR2 is also reasonable. DDR may be acceptable if we can give someone at least 1 Gib. Computers with 1+ Gib of ram will work for most basic tasks, but if we're going to spend a couple hours building a machine, we might as well do it on a better machine.