upgrading the turbo requires (to be effective anyway):
turbo back exhaust with a bellmouth downpipe
a custom tune
You might also consider:
EDIT: You CAN just slap the turbo on there provided you tune for it. This would be rather stupid though. The stock intercooler on the 06-07 WRX is the smallest of any year WRX. There's gains to be had replacing it on your STOCK turbo, not much point in going to something bigger with it still in there. Fuel system is similar. As always, a custom tune for any non-stock condition is a good requirement.
2015 STi: Still Stock
04 STi, 04 WRX: SOLD