Front and bottom as intake while rear and top as exhaust.

RM 800 can get high end AIO water cooler already, if you are going for air cooler, no need RM 800.

i5 6600 is a locked CPU (or did you use the Skylake non-K exploit to oc it?), any sub-RM 200+/- air cooler will do the cooling job just fine like CM Hyper 212X or Cryorig H7 or H5 Ultimate.

Difference is quite significant, I tested i5 4460 few months ago with Corsair Hydro H100i and CM Hyper 212X.

H100i is able to keep the CPU < 40°C all the time when during max load compared to 212X's ~55°C, 55°C is still a very good reading though considering the weather in Malaysia is so hot.

If you are willing to spend so much just to get the extra ~15°C, then go for H100i V2 for 240L, H115/NZXT Kraken X61 for 280L, the rig looks nicer too with AIO liquid cooler 

It depends on what is your priority.

If you install at front, the fans on the radiator will take fresh air into the radiator, CPU will be cooler while the rest of the components will suffer due to the increased heat dissipated from the cooler.

If you install on top, the radiator will suck hot air out of the case, CPU will get those warm air while the rest of the components will be cooler.

This might create strange/negative airflow 

BTW Kraken X61 is 2 x 140mm.

Actually you don't have to worry so much on this, the difference in delta temperature is just few °C for intake/exhaust radiator setup.

You can go for push and pull setup on radiator too, just like what we can do with air cooler now.

Just get 2 more 140mm casing fans and install on the other side of the radiator which doesn't have fans installed by default.

Yup, total 4 x 140mm fans.

I think it's better to pull if you're lazy to clean it in the future. The push method will also push dust onto the radiator making it stuck there. Unless your rig's having a really good positive airflow(which doesn't suck dust into the casing), you're better off with the pull-method.

If install on top then most likely not enough clearance, front still possible.