Lab computers: high end now or average system and repurchase in a couple of years?
In general, I would argue in favor of getting the best available system that fits within your budget at the time of the purchase. You might be able to save some money now by buying a less powerful system, but the danger of doing so is that you can't necessarily guarantee that in N years, when you plan to buy the replacements, that you'll have the money in the budget to do so. Long-term funding guarantees really aren't, and what is promised today might not hold up several years from now.

By getting the best available system now, you postpone the "must replace" date a while, which can give you more time to find a replacement (or the funds to replace it, as need be). Also, if your money is time-restricted in its use (you have to use it this year, for instance), there's no real incentive to skimp.