|"I downloaded an app, to tell my specs but it did not include the PSU"|
All you need to do is open the case & get the info off the PSU label. ETON is not a brand I'm familiar with, but we need to know the wattage & amperage specs. Also, why are you only running a single stick of RAM? You're taking a performance hit by running single channel mode. And you should consider adding a graphics card so that you can get off the weak Intel onboard graphics.
As for overclocking - the default clock speed for the E7400 is 2.8GHz with clock settings at 10.5 x 266MHz frequency (1066MHz FSB). For best performance with a Core 2 Duo rig, the RAM frequency should match the CPU frequency. SO, to overclock "properly" you should keep the RAM at 400MHz, raise the CPU frequency to 400MHz, & lower the CPU multiplier to 7.0. That will keep the CPU at 2.8GHz but the FSB will jump up to 1600MHz rather than 1066MHz & the RAM freq will be running in sync with the CPU freq. That's known as maintaining a 1:1 ratio. Once you get the ratio straightened out, all you'd have to do is try increasing the CPU multiplier. It should easily run at 3.2GHz (8 x 400MHz) & probably wouldn't even require a voltage increase.
There are numerous BIOS settings that would need to be changed to pull it off. If you're interested in giving it a try, let me know & I'll post a list of settings.