Personally I would spend about $1500.00 -$1800.00 on a custom built with the expecation of doing another in 2-1/2 years
yes / photo shop is the only app I have seen that really uses four cores
the remaining amount you will have saved will put a major dent in a new one at that time
NO computer can be expected to last 5 years / unless you are willing to use it while its slow for the last two years of that stretch !~