Buy a machine that originally sold for a higher price whenever you can. Consider buying refurbished or repaired units of current or recent models. Think about online auctions. Assume that upgrading today's computer in four years will be equivalent to converting a tractor into a race car. No manufacturer is highly rated in all three categories and only a couple are respected for both computers and monitors.
Savings are small and artificial.
Avoid processor and computer brands that have been discontinued. Ask about return policies and restocking fees when considering a computer that isn't sold new in a box. Expect the recent computer industry decline in the quality of building and parts to boomerang eventually, but don't wait for that to happen. Most chain retailers will give you a price adjustment if the price on a product is reduced within a week or two of purchase. Make sure you get a warranty on a computer that isn't sold new in a box.