The Phenom most likely will not clock to 4GHz on safe voltages. 3.6-3.8 is the norm. After a certain voltage the amount of cooling you have means nothing. Excess voltage, even in conditions where the cpu never gets hot, will also kill the chip. It's called electron migration. Basically, voltage "leaks" from the CPU's transistors, eventually breaking down the insulators between them. Once the insulators break down, the chip becomes unstable. But, with OCing, nothing is guaranteed. It's not even a guaranty the PII can hit 3.6.
The amount of voltages I've seen every Phenom II take to get to 4GHz or more is much too high to be safe on a 45nm chip. It would only be useful for shorts stints for benchmarking, not for 24/7 use.
The Q9550's clock to those speeds on much lower, and therefore safer, voltages. The crappier chips can usually get to 4Ghz on 1.4V. Newer steppings require even less. Perfectly safe for Yorkfield. The downside is, you need a board that will hit 470 fsb with a quad. P45 boards usually do this, but again, just like any OCing, nothing is guaranteed.
If 4GHz is something you must absolutely achieve for 24/7 use, the Intel has much better odds. (But again, I must stress, no overclcoking is guaranteed) If you are willing to settle for 3.6 - 3.8 for 24/7 use, the odds are about even for both chip.
As far as your case choices, the side vent doesn't matter. A fan most likely won't fit there with the cpu's heatsink anyway.
I might buy the CoolerMaster with the side window and drill some holes in the window myself!