The comp originally came with a 65nm Windsor 4200+ cpu, and the E3300 Celeron is 45nm. Their both rated 65w so I'm thinking the power use would be equal or even less. The only variable is the GT 240 that's rated for 300w. The consensus from my searching of the forums seems to be that an upgrade to a $30 or $35 psu would be a good investment, I'm just not sure how long I'll keep my computer before I do an entirely new build.
This is my first time moddifying/adding parts/building any computer and I can say that choosing/finding the best deals/researching is intense. It's fun though!