You really just want lots of ram and a really fast processor. Also the civ engine is not multicore optimised, so favour maximum power in a single core rather than rspread over multiple ones.
Problem is, you basically don't get any decent single cores anymore, and the top-of-the-line processors usually carry a hefty price tag.
Keep also in mind, that as a rule of thump 30% more processor speed are
just noticable.
I have an older Thinpad with a 2 GHz Core2Duo, 2GB RAM and a dedicated low end video card (ATI X1400, 128 MB RAM) and can play CIV just fine with a bit reduced graphics settings.
Stay away from atom-based machines if you plan to do serious work. Generally avoid integrated graphics (anything with "up to xxxx MB video memory" if you want to do some gaming. 2 GB of RAM is plenty enough, if you can get more without paying much more, get it, but keep in mind that no ordinary windows software can use more than 2GB, much less profit from it significantly!
Video card performance can differ by orders of magnitude, but it can be rather tricky to find out how a specific model will perform as the manufacturers like play all kinds of dirty tricks with this, basically you need to go through some competent reviews. Most importantly: will you get regularly updated drivers from the manufacturer? Check their website for older models! It is usually quite tricky and sometimes outright impossible to get a generic ATI/Nvidia driver to run on a laptop.
If you can afford it and don't need much storage space, go for a SSD. It's downright amazing how snappy everything is compared to even a desktop HDD, let alone a notebook HDD.
If you plan to drag the laptop around a lot, keep an eye on the weight, should definitely be below 5 lbs, so you should prefer a 14" model.
If you are going to use it extensively and/or move it around alot, you should seriously consider a "business notebook". These are usually better designed and more robust, and you can expect a better treatment at the hotline

The downside is usually a "now frills" approach and a sleightly worse price/performance ratio.
My personal recommendation would be either a Thinkpad T400 or an equivalent Dell business notebook. For one of these an extended warranty might make sense, as Lenovo and Dell tend to have efficient service that actually is capable of fixing a broken notebook.
Good reviews can be found at
http://www.notebookreview.com/reviews/.
Specific infos on thinkpads:
http://forum.thinkpads.com/