But erm could you measure the space between the motherboard and the side panel. I've got a scythe Ninja 3 with two fans on low as CPU cooler and run a i7 2600K at 5Ghz. I ran Prime95 for 4 hour’s and the temperature got just into the 60C which is great so I'd suggest any one the Ninja 3 as cooler.
If the space between the motherboard and the side panel of the case is more than 160mm then you can almost fit any heatsink.
Scythe MUGEN-2 RevB is good
Cooler Master Hyper212+ is good
Be Quiet Dark Rock Pro and Be Quiet Dark Rock Advanced are good
Noctuna NH-D14 is good
prolimatech Armageddon with one or two 140mm fans is also good.
Just a few suggestions, the more expensive the cooler it will run.
You don't have a rear 120mm exhaust fan?, the H50 is designed to mount replacing that fan, you may be seriously inconvenienced trying for a side door mount, but if you have no other choice it may be your best solution.
You do need to keep in mind pulling the stock cooler and switching to the water cooling solution removes the airflow over the voltage regulators, and you may need to mount a small cooling fan positioned to cool those.
The Raidmax Sagitarius has more room inside than 6", outside measurement is 8.25", are you saying you only have 6" inside to clear the side mounted 120mm cooling fan?
It also looks like you do have a rear 120mm, so you could cut out the rear mesh, (sometimes a little case modding is required) and install the H50, and use the existing side 120mm to blow straight onto the voltage regulators.
The rear fan is right by the power supply though and it doesnt have enough room as it'd hang out of the case if i put radiator/fan on the rear fan thing as isn't the radiator longer than the 120mm fan? and it'd prob hit the power supply there, unless i move the powersupply into the bottom set of the HD bays.
I can hardly fit the heatsink i have now on my core 2 duo set up in the case and the heat sink is 131mm x 108 x 153 only.
That's what my current core 2 duo setup looks like with the tower 120 inside and as you can see the 120 fan on the back it right on the edge of the case, unless you're telling me to install the radiator/fan on the outside of the case after i cut the rear mesh out.