As for the CPU, it does make a difference for Civ AI turn times. The 3350P isn't top-end, but it's decent and a good price. I'd probably go for the 3570 over it, though, given your budget.
There is no reason to not buy a Haswell chip if you want a top end machine right now. With Haswell also comes the LGA 1150 mobo which, if Intel continues its current business model, should be good for at least one more chip release.
Do yourself a favor and build a machine from scratch. That way you only get what you want/need and save a ton of cash.
For example, a "high-end gaming computer" from Alienware cost over $3000 and pales in comparison to
my ~$1300 custom rig.
I agree that custom building is better for the money for a high end machine, but there is no way in hell your $1,300 system is even close to that Alienware build. The CPU and the video card alone are orders of magnitude slower than the Alienware build, even if the pre-built is sandy-bridge-e and not Haswell. I agree that you could put together a great build for $1,300, but picking the same parts as that Alienware still yields a fairly expensive build of around $2200-$2500, depending on how choosy you are with components. A quick Newegg shopping trip turned out a build with the same base components (GTX 690, 512 GB SSD, 16GB RAM) with an i7-4770k instead of i7-3820, since it is better, for around $2300. Give or take a few hundred to choose higher performance SSDs or RAM or whatever. Now of course, cut out the wastes of money, like the 690, and pick a smaller SSD, and you are back in reasonable-price land. But still a bit higher than $1,300.
Anyways custom built vs pre-built is usually more about convenience. In the case of Alienware they also throw in gimmicky stuff (like the moving case fins and changing LEDs) that some people like. If you're thinking pre-built you are usually willing to spend the extra money to not have to worry about building it yourself. Even though 90% of the time a custom build will go totally smoothly, if you have never done it before it can be sort of intimidating.