That's for the theoretical business. Now for the practical questions.

Will running two 8800GTSes in one machine make Poser 6/7 and Vue faster?
Probably not. Two graphcis cards working in tandem only has proved (somewhat) beneficial in games. I'd not spend money on a second graphics card.

The case already comes with 3 fans, that should be enough. My Antec Performance One P180B also has three 120 mm fans built in - it's the case that holds the Q6600, 8 GB of RAM, a 7800 GTX graphics card (hot!), 4 harddisks and 2 optical drives. The case is cool and quiet.

CPU fan: Yes. Fast CPUs definitely require forced cooling. But should you buy a CPU cooler or not? That depends on how you order the CPU. Often you can order the CPU as a "boxed" version or a "tray" version. "Boxed" means that the CPU comes with a fitting cooler, "tray" means CPU only and you have to buy a separate CPU cooler.

The cooler that comes with the boxed version of the CPU does the job, and does it well. You can opt for a seaparate more expensive cooler, in order to have a more silent cooling solution, or to have more rigorous cooling (important when overclocking). From my personal experience the "silent" coolers aren't as silent as advertized - not worth spending extra money on. 
An advantage of the boxed version of the CPU is that you don't need extra cooling paste. The cooling paste is preapplied onto the provided cooler. If you go for a separate cooler, you also need cooling paste.

Cooling the RAM is not necessary, unless you want to overclock. Which I do not recommend.

So as far as cooling goes, I recommend buying a boxed Intel CPU, and that case with three fans. No extra cooling needed.

The pen is mightier than the sword. But if you literally want to have some impact, use a typewriter

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