If you go to the 80Plus Certification website http://www.plugloadsolutions.com/80PlusPowerSupplies.aspx (or the Wikipedia page) you can find the efficiency table (show below) that describes the level of efficiency under each tier of load. Based on your previous unit's efficiency rating, which is probably lower after being used for so long, you would essentially be gaining the difference between that advertised rate and the certified rate of the new unit. 
 
On their site, they have a descriptive pdf on the testing process and how they calculate efficiency so you can see what specifically they test. Pretty much it is the sum of power output divided by the sum of power input, but they mention this as 'real power' so that would imply they are making some adjustments from nominal readings; though for most intents and purposes you can probably ignore this.
 

 
So roughly, at 72% efficiency you're getting 540W out of every 750W you put in. => .72=540W/750W
 
If you increase your efficiency to 80%, you would only need 675W to pull the same 540W of power; which would save you 10% in required input power to run the same system. => .8=540W/675W => .1=(750W-675W)/750W
 
At Titanium (90%) you would need 600W for that same 540W which would be a 20% decrease in required power to run the system.
=> .9=540W/600W
 
Of course these numbers are ideal, but for demonstrative purposes I think it gives a reasonable description of the process. 

 
Just go with the Evga 1300w  and you'll be happy like I am.       =)

I'll never use that much power, and didn't have enough money for that so I went with the 750.