Replacing stock heatsink

You could get away with getting rid of one of the 120mm fans on the side panel to fit a heat sink if you had too. Since your not Overclocking you dont need anything to crazy but i wouldnt skimp too much the I7 puts out a ton of heat at stock speeds also the cooler you keep the better the turbo mode on it will work.

If you really want to keep that fan on the side this one should fit nicely and still give great cooling Noctua are well known coolers and always do great in reveiws.
Noctua NH-C12P SE14 140mm SSO CPU Cooler
http://www.newegg.com/Product/Product.aspx?Item=N82E168...

Here is a link to a recent Tom's Hardware article with useful information about 10 popular cpu heatsinks:

http://www.tomshardware.com/reviews/lga-1156-heatsink,2...

I know the article states Intel LGA 1156 heatsinks but just about all of them are also compatible with Intel LGA 1366 cpu's

Here is a link to a very good web site with more useful information about cpu heatsinks that will help you make an informed decision:

http://www.frostytech.com/top5heatsinks.cfm

You can use the Google embedded search feature at the web site to find more information about heatsinks for specific cpu sockets.