The most important thing to consider is what part of the computer you are stressing. If you are writing scientific processing apps, then you are clearly stressing the processor. If you are doing GIS (Geographical Information Systems) work then you are stressing the IO channels. Other apps fall in between.
If you are doing CPU work then the calculations are trivial. If it's data heavy work then a few minutes with manufacturers brochures will give you a good idea of what to expect.
But you are probably asking about the in-between stuff, like website subscriber databases. That's pretty much black magic, since you are working with a pretty large amount of data and doing a little bit of processing on it. Looking at the data transfer specs for your machines will give you a good idea of the maximum performance you can expect. But like others have obviously told you, you just have to suck it and see.
I didn't believe in evil until I dated it.
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