If like most of us you will be tempted before 5years are up.
Can see no real reason to worry about updating as you suggested if your limiting your self to a £1000 for purchase. Bar to say that i would consider at time of purchase to have the XP 2800 OR XP3000 for £50 to £100 more at this time.
If you wait untill later as you suggest, the purchase of such upgrade would leave you
with processor no longer required.