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Go all out on the case and moniter as these have the longest upgrade periods.
For everything else, try to buy stuff between 3-6 months old. Nothing bleeding edge, but not last years model either. Seeing as you posted this in a geek forum, suggest trying to get as generic hardware as possible. The Asus MB, SB Audigy Card, The Linksys Network Card, whatever. Like everyone here you probably try an assload of OS's over the span of your computers life so building with hardware that's more likely to have drivers built for it will make things easier in the future.