Coming at this from any other direction will simply get you in trouble. The time to buy a computer is when you need it. You can't get the fastest computer there is. If you wait for it to arrive, another faster system will be on the next truck and you'll never buy a computer. By the way, prices for existing technology always go down.
New technology comes in at the old price.
The same rule applies. You're going to want another one. Advances in software, hardware and your own sense of anticipation will make today's fast computer appear old and tired long before you wanted. So, plan an upgrade strategy before you buy in order to extend its useful life as long as you can. 9 Conversely, the only valid reason to upgrade software or hardware is to solve a problem you can't fix any other way.