So I have a few different PC's I have played Civ on:
including:
i5:4460
i7:2620QM
a couple of core2Duos (as well as a laptop one)
These have been paired with anything from a geforce 9600GT to a geforce 630m through to integrated intel graphics. My latest PC desktop has a 970GTX.
I've also loaded from SSD's and original hardrives.
The game isn't exactly graphic overload and even the 630m (about the same performance as the 9600gt) was able to play with AA and high detail setting (except perhaps leader scences?).
The SSD made no difference for Civ.
It's all CPU. The faster the quicker. My i7's are a few generations behind the i5, so a comparison isnt' fair, but as far as it is, the i5 is slightly quicker than the i7 (10% - 20%?). the multi-threading doesn't appear to be great. If I had a choice, I'd go for the quickest clock speed (MHz) and not worry to much about number of cores if the lower core enables higher clock speed.
on the RAM front, my new PC (i5) has 8GB, the i7's have 16GB and the core2's have 4GB. I have tested the i7's with 8gb and 16gb. It made no difference for anything (which is why i saved a few$$ and only got 8 in my new PC). Hard to say if 4GB would struggle, since everything else in those PC's are bottlenecks.
It's a 32 bit application though, so it uses only 2GB roughly max.
