Prioritizing Computer Replacements?

cuzality asks: "The public library I work for is (hopefully) going to get some money in the coming months to replace some of our computers. Since we can't replace all of them at the same time, we need to decide which machines to replace first. We have begun working on a survey instrument to help us accomplish this, but since this is the first time we are replacing computers (most were bought in 1999), this is also the first time we have tried to put together a survey for this purpose. What kind of criteria have you used to prioritize replacement of computers? Examples of surveys you have used would also be very helpful..."