Well, that's true as far as O/S goes. But since most computers are sold with 4GB of RAM, that's 1 more GB than a 32-bit O/S can use, so those users are just hurting themselves.
But people still have 32-bit, single-core processors? Really? They must be at least 5 years old. I think anyone with a machine that old can be expected to upgrade to play a game released in 2010.
Actually, no. All of Intel's Atom processors targeted at the mobile market until two months ago - the ones that really give you long battery life - are 32-bit only. There are some 64-bit Atoms, but those consume considerably more power. So you've got an awful lot of people who bought 32-bit netbooks in the past year. Granted, most of these people probably have other computers with 64-bit support, but with Atom, Intel's done a good job of extending the 32-bit lifespan.
Pentium 4's aren't in distribution anymore - that ended in 2008 - but there's still a lot of them in commission, and most of them aren't 64-bit. As for Windows 8 being 64-bit only, believe it when it's announced. Legacy applications drive a lot of Microsoft's business, and may well persuade them to keep 32-bit support again. It's the same reason that you can still get Windows XP (as part of Windows 7, no less), that sales of Windows 3.11 Embedded ended just last year, that IE5 is still supported (and IE6 will be for 4 more years), and that the DOS-legacy line of Windows survived for 7 years on its own after NT came out before XP merged them. Perhaps Windows 8 will be 64-bit only and will come with a 32-bit XP Mode in the Professional version, but I wouldn't be surprised at all to see a 32-bit version.
I'm not getting my hopes up until there's something official, but I'd love to see both of these. Although the turns play out sequentially in Civ, there are likely tasks within each AI turn that can be parallelized. Galactic Civilizations II did multi-core and came out half a year after Civ4, so Firaxis should have had enough time to figure something out by now.
64-bit would be really nice, too, if Civ4 is any indication of the memory consumption Civ5 will have. It might even be enough to convince me to switch back to a 64-bit OS (I reverted to XP 32-bit after a couple months of dabbling with 64-bit as the compatibility was better in XP 32-bit). Really, if memory consumption per map size were more along the lines of Civ3 than Civ4, 64-bit support would be a non-issue, especially with the 3 GB switch, but as we can't expect memory consumption to be lower than Civ4, 64-bit would be really nice.
Can't wait I have enough to buy it in 3 more weeks. 
