11.1 inches? Wow, that is going to be tight! Do you own a
Dremel tool, just in case?
3600 Mhz is plenty fast - that is a decent overclock. It sure is fun to see how far you can go.
When you settle down, you might want to play around and see where the sweet spot is between performance, temp and voltage. I have an i7-920 with the same base clock speed, and run mine at only 3300 Mhz for an everyday overclock.
If you want to pursue extreme overclocks, you will definitely be stressing your power supply, and should think about an upgrade. Enthusiast computing does not follow the rules of normal users, and you should have excess capacity, if nothing else, to keep your power stable.
If, however, you just want you machine for regular use, the Antec Supply Calculator is pretty conservative, and your actual power consumption is likely much less. For example, most of time you are not going to run your system at 90 percent utilization 9the recommended default), but maybe closer to 70 percent under heavy usage. Just don't go crazy on your voltages or run Prime95 for 24 hours and you should be OK.
Also, Antec is a reputable brand, and is not likely to fry your other components if it fails. If it is very important for you not to suffer down time on your system, you could buy another PSU and keep it in reserve. It can always go into your next build, and won't go to waste.
When you decide,
here is the search I use to find a PSU.