Fred,
Keep in mind that besides the voltage, the frequency is also different in Europe -- 50Hz there, 60Hz here in the States. My iMac is newer than yours, but it has the electrical info printed on the back of the display -- check yours to see if it is there too. I believe it's required by international law to have this info visible on all electrical appliances.
I am not sure how computers differ from other appliances in this regard, but I do know that electrical alarm clocks, for example, use the electrical current's frequency to keep the time. My European alarm clock did work in the U.S. on a voltage converter, but the time was WAY off -- like ten minutes every hour. It would stink if that happened with your mac. Please check this too.
That said, if it turns out to be simply a matter of converting the voltage, then I don't think you need a step-down converter with such high wattage (750W). Since converters get more expensive (and bigger) as the wattage goes up, you would be well-advised to not go too much higher than what you actually need. According to the Apple site, the new iMacs have a maximum continuous power of 180W. Getting a 200W converter would be cutting it a little close, but 300W should be more than plenty. Just don't plug your toaster in there too... 🙂 (Tip: anything that generates serious heat, like coffee makers and toasters, requires a high wattage converter.)
However, if you have peripheral devices (external hard drive or whatever) that you also want to plug in to your power strip, you may need to go higher. And do get the converter before you go; you'll have much more choice at much better prices here. I used www.voltageconverters.com and was happy with their service.
Hope this helps.
