Wattage is the product(multiplication) of Volts and Amps - without knowing the detailed info on the psu it's anybody's guess. For a psu that old it's time to consider a replacement. (a quality one)
Input AC Voltage is not a factor in determining the output wattage capabilities.
For example a PSU that can accept both 120 VAC and 240 VAC will have the exact same output wattage capabilities. What is affected by the input is the current drawn by the PSU for a given output wattage. @ 120 VAC the current drawn will be double that when the PSU is placed on a 240 VAC input.
The OUTPUT wattage should be on a label on the PSU. If no Label, look for a brand and model number for the PSU and do a Google search.
As dish_moose indicated, if the PSU is 10 years old, it's time to retire it. Newer PSUs have 1) improved on efficiency and 2) increased the +12V rail capabilities as Newer Computers have increased the load for the +12V rail, while decreasing the +5V and +3.3 V requirements. 3) Electrolytic capacitors age, especially if stored over a long period of time - When they go - the PSU goes PUFF.
"Actually, 230v is equal to 450watts. simplest answer!!!" NOT TRUE You look very silly (being polite) answering a question you obviously know nothing about. Listen and learn and do not mislead other people by posting something that is completely false. What you said is like saying 50 M.P.H. is the same as 50 gallons.