I have two thoughts that I use when buying hardware:
Always get the most expensive thing I can afford when I'm making the purchase.
Always assume I am never going to upgrade the machine I buy.
Details:
1. If you have a budget of x dollars, spend the budget. You'll never complain about having too much power or speed. If you are realistic about what your budgets are, then you won't overspend. just be aware of what you are getting.
remember that "most expensive" really means best price/performance. If i had $2,000 to spend on a render only box, i could probably get better performance out of AMD than Intel at this moment. Thay may change down the road. If I had $50,000 to spend on a whole render farm, it may be different. I may be able to get a larger quantity of slower chips.
- There is this myth, oh i can just upgrade my machine when the new chip comes out! First, when was the last time you actually "upgraded" a machine's processor? I have been using PCs since the 1989 and I have never simply swapped out a chip on the same motherboard. The reason is that the cost of getting that 3GHz P4 is going to be a lot, and then you're stuck with the 2.25GHz chip that is totally useless.
A much better approach (in my opinion) is to buy a whole new machine when you want that new chip. That way, your investment in the old chip is not wasted, cuz you can still use it. Either as a second workstation or stick it into your render farm.
Obviously, this is talking about chips only. Addig RAM or getting a better graphics card is a little different, though with the current prices of the highest end graphics cards, I would probably treat them more like CPUs. On the other hand, it's probably easier to sell a secod-hand, high-end graphics card than a used CPU.
Course, these are just my thoughts.