it depends on how much your willing to invest.
The main goal with fsx / p3d is to push as much data through the cpu as is possible, and there are a few ways to do this.
1. buy a modo uber expensive cpu with a lot of cores and put eight to sixteen gigabyte of memory in there for it to pull from
2. buy a normal 2,4,or six/eight core cpu, and back it up with a three board sli array and cuda ( matrixing in over 900 cores ) and then add eight to sixteen gigbyte of ram
3. (what i plan on doing ) buy a multi processor server MB capable of at least two i7s or 9s, add in 32 gigabyte of ram, toss in your favorite graphics card, and have fun..
with the detail and sophistication implied by the direction that flight simulation software is taking, more powerful ( not neccessarily faster ) machines will be required. Those same implications also show that that sophistication and detail is growing at an amazing rate, requiring a machine that is capable of some certain amount of longevity as it grows. Thats the main reason all my recommendations seem a little over the top. you want something that will be able to handle whatever comes at it over the next five to seven years.
another extremely important thing to consider is cooling. Heat not only destroys a machine, it slows it down.. it makes it harder for the machine to do what you want it too.. Fans simply will not work well.. they're ok for a grade schooler typing a math report, but for serious graphics and cpu throughput, you need something a bit more powerful. I've used water to cool mine for the last six years, but, it isnt enough with these new games and graphics capabilities. Liquid nitrogen is still not realistic ( and costs a fortune ) but, there are micro refrigeration cases available that will keep your cpu and system at a nice and balmy near freezing temperature, allowing the machine to work at optimal efficiency.
Pam
OH and, dont fall into the speed trap.. dont pay attention to the gigaherz of the cpu. although its important, its not as near important as the ability to perform operations. Lets say i have a 2.4 gigaherz cpu. it can do just so much work right?? some people will tell you that its because its only a 2.4 gigaherz cpu and you should buy a much faster one ( and pad their wallets with income ). BUT, lets take that 2.4 gigaherz cpu and add a second one.. hey, they're cheap these days right?? so now, we have a machine that runs at 2.4 gigaherz, but is doing twice the work of a single 2.4 ghz cpu. add more cores and the computational power increases exponentially. and your still running inexpensive, slow cpus, but doing more work per second than the expensive fast cpus.
check this page out.
http://www.nvidia.com/object/tesla-supercomputing-solutions.html
