depends upon your board. You may need to rotate it 90 degrees. I know my true 120 on a p6t deluxe v2 would work in push pull with 6 dimms of ram venting towards the rear of my case. I do use 25mm fans versus 38mm fans.
standard sized memory modules (no tall heatspreaders) should fit just fine. But populating all 6 memory slots will strain/burden the memory controller, this isn't a problem for a system running at stock speeds. But it's not recommended for overclocking.
No, on the i7, i5 and i3 cpu's the memory controller is built into the cpu. Is there a reason why you want all the slots filled? The i7 with 6GB (3x2GB) of memory creates a tremendous amount of Memory Bandwidth, only a very few programs will benefit from more ram. For 95% of all users, 6GB (3x2GB) is more than enough (a guesstimation). My sytem with 6GB (3x2GB) at 4.2GHz will outperform a system with 12GB (6x2GB) at a lower cpu speed everyday (or it has up to this point in time)
More is better usually, but there comes a point when more isn't being utilized and the cost of having it available is less performance on another component. You'd see more of a benefit if you weren't overclocking.
I have OCZ memory in both of my i7 PC's
I'm assuming this is for a gaming machine? The only people I know that use more than the "standard" amount of ram you see 90-95% of people use like Slaymate says are people that use heavy-duty design/modeling/and/or analysis software. Even so, they use Xeon or server processors rather home/business CPUs like the i-series.