Purchase self justification is all that is needed.
If you can justify spending £900 on one PC component, go for it. If not, then drop down to the next best at ~£300 which is an MSI 7970...If that still seems too much, then you have the 670 and 7950. All will cope nicely with todays current games (except maybe Crysis 3) and you shouldn't have to alter settings too much for your 1600P monitor
Fixed that for you