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Keep in mind if your room is really small and has poor ventilation the room will heat up. Under normal conditions the room should maintain the temp it was at when you turned your Mac on.
 
Here's some actual power consumption numbers for a fully-loaded 27" iMac (3.4GHz i7, 16GB RAM, 256GB SSD+2TB HDD, 6970M 2GB VRAM) *and* a 27" LED Cinema Display. An Apple Wired Keyboard is plugged in, an iPhone 4 is plugged in and charging, it's connected via 802.11n, and bluetooth is enabled (Magic Mouse or Razer Orochi).

iMac only, basic use, 50% brightness: 95W.
iMac and display, idle, 50% brightness: 120W.
iMac and display, basic use, 50% brightness: 145W.
iMac and display, compiling and light gaming, 50% brightness: 180W.

These measurements are consistent from both an APC Back-UPS RS700 and a P7 Kill-a-watt.

I think a significant portion of that is coming from the extra display. If I remember correctly, load my 27" iMac was somewhere in the 100-120W (3.1GHz i5, 12GB Ram, 1TB HD, 6970 2GB)
 
Yeah

My 2009 Corei7 27" gets warm under load - but part of the problem is I've got another 23" lcd, another 22" lcd, and another three computers all in this room.

Cant be helped.

As warm as it gets, it's not as warm as my old G5 2.5 dual - that thing was like a space heater with a mac
inside it. My 20 year old Maine Coon cat loved it - he'd lie next to it in the winter to stay warm.
 
Thanks for all the input, everyone!

My Core 2 Duo PC + Nvidia 9600GT is like a space heater, especially when SETI@Home is crunching numbers. Seriously, you sit down at the desk and you can feel the warm air blowing onto you.

My Core Duo iMac doesn't seem to affect anything.

Contrary to what someone else said, the halogen floor lamp in the room does substantially warm things up. There used to be part of a spider web on the ceiling over the lamp, and after turning on the lamp you could see the web blowing around from the rising heat. On a cold night with the door closed the room would get nice and toasty from that light and the computer after an hour or two.

Anyway, you guys helped me make my decision; I ordered the i7 iMac yesterday!
 
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