You can find some help on getting relevant details off the power supply at this site (about half-way down the page:http://www.upgradevideocards.com/criteria.html
Depending on how the PSU is mounted, the label might be turned away from view and require taking it out to see the label.
Alternatively, did the ordering information list the make and model of the PSU?
By the way, I could not find any details at iBuyPower and the listing on newegg for that computer, using that exact model number - 918i - shows it with an HD 5450 video card. Are you sure you have the HD 5750? Although they sell a lot direct online and might allow some part substitutions within the same model number, something the large manufacturers normally do not do.