Direct X is a more significant issue for Civ V because Firaxis has specifically said DX11 will be supported. DX9 will be fully supported, and if your computer is a DX9 computer, you will be able to play Civ V. It'll look good; you'll almost certainly love it.
Those gamers lucky enough to be on Windows 7 with brand-new, DirectX 11 compliant graphics card will get some extra graphics fireworks. Nothing essential to enjoying the game, probably just some better looking water, more vibrant colors, perhaps slightly better performance when scrolling or looking at large sets of units.
CAVEAT: the screenshot I'm about to link to is from a preview build of Civ V and doesn't represent the final product, plus I don't know anything i'm an idiot
grain of salt.
Here's a screenshot showing the some of the differences between DX11 and "integrated" (probably DX9) graphics on an early build of Civ V.
I asked for advice on the upgrading from DX9 to DX11 question not too long ago on this very forum. Sage advice received: wait for the game to come out before buying new DX11 stuff. DX9 will work; you'll be able to play the game, and you can always buy new DX11 stuff later.
On 32- vs. 64 bit computing. In this context, 32- or 64-bit refers to the amount of random access memory (RAM) the program can use at one time. Civ V will almost certainly be a 32-bit only game. Don't worry: if you have either a 32- or 64-bit version of Windows and a computer that meets the game's system requirements, you will be able to play Civ V with no problems. You will be able to play any map of any size. You will be able to have the same number of units.
To be as clear as possible because this is confusing:
32-bit and 64-bit versions of Windows can play Civ V equally well!
The vast majority of PC games are 32-bit. They're doing great. 64-bit PC games are still science fiction. Civ V almost certainly will not have a 64-bit version [EDIT: I am wrong about this -- see below]. This is not a problem. Civ V will run in 32-bit mode on your 64-bit Windows box, in the same way that your 32-bit web browser is reading this article on your 64-bit Windows box.
My point is: don't worry about 32- vs. 64-bit for Civ V. It's a non-issue for the game.
EDIT: Firaxis has confirmed that they will provide a 64-bit version of the game. Please scroll down for more information.