If we're going to start the wild speculation game, here's my $0.02:
2 GB is decent for Civ 4. Most new computers ship with at least 3 GB and computers labeled as "gaming computers" ship with 4 GB. 4 GB gets tossed around a lot as a good amount for Win 7 so I'm guessing you'll want 4 GB RAM in your computer.
Processor speeds seem to be hovering between 2 and 3 GHz with anywhere between 2 and 4 cores, generally speaking. Will Civ 5 utilize multiple cores? Will it be CPU intensive? Who knows! I'd suggest at least a dual core, 2 GHz CPU to be safe though.
Video cards also come in a wide variety of flavors and, as V. Soma said, I'm guessing this is where you'll want the most power. A 512 MB DDR2 video card will probably be good enough, but I won't even take a guess as to which of the dozens available will be a good benchmark. Onboard video will definitely be out of the question. DX 10 capability may not be required, but it is a good benchmark for making sure you have a video card capable of handling a newer game.
These probably won't be on the box as minimum specs, but I wouldn't get any computer nowadays that I want to use for gaming that doesn't meet these specs as the experience would be pretty painful.