Hi,
The wattage on a PSU is the maximum it can supply.
This doesn't mean to say that if you have a 1000 Watt PSU the output is 1000Watts all the time.
The system draws the current it needs.
So, for example, if you buy a Video Card for your system and the Video Card specs suggest that the maximum wattage the card will need is 200Watts
( this is not the PSU but the power required to run the card ) then your present PSU would not be able to supply the required current to run the whole system.
A 1000Watt PSU could ( with some to spare ).
It supplies the system requirements as and when the system needs it. ( The current supplied varies as you are doing different things ).
Current is measured in Amps.
Watts = Amps x Volts.
The PSU is generally bought to comply with the requirements of the new Video Card.
You should be able to look at the card specs and the system specs will normally specify the minimum requirements from a PSU.
If the specs suggested 450 Watt minimum then there is no reason why you could not buy a 750 watt PSU.
I hope this helps.