Buy the computer that meets your application.
If backtesting is what you want to do... get the most powerful machine for it.
Note the qualifier "FOR".
"Powerful" doe not mean the most powerful hardware on the market.
First thing to check is to see if your software is a power hog.
Most people assume backtesting scenario requires lots of power. But you'd never know. I don't assume. I have done backtesting on option trades and it require very little power. Don't take advice from people who does not know how you operate. (including me, LOL).
Second thing is to check to see if your software utilizes multi-core.
If yes, get a quadcore, if not, a dual core would do fine. (nobody buys single cores anymore.)
Don't assume that more memory is better.
XP can read 4GB of memory, but the top 1 GB is reserved for the OS. THerefore the most you get out of 4 GB is a 3GB (+/- a bit, depending on your hardware/software settings).
(You will notice some notebooks offer 3GB memory. THis is the reason.)
At the same time, don't assume that more memory does not help.
Read your software manual.
A lot of people (eg Gnome) are still living in the dark ages. He believes 1GB is all you need. He has never tried the new breed of software, nor does he understand there are trading methods/setups that uses more memory than 2GB.