This isn't a big problem, of course, since XP is not meant to be a server operating system. But it's still mystifying to people who try the /PAE flag and can't figure out why it doesn't work. (来源：EnglishCN.com)
Oh, and just in case you for some reason still wanted to try PAE: It eats CPU time, too.
(You can read more about boot.ini switches as they pertain to memory and driver breakage on Microsoft's page for driver developers, here.)
As things stand at the moment (June 2007), the sensible course of action for the vast majority of Windows users buying a new PC is to get a standard 32-bit WinXP system with at most 3Gb of memory. It'll come as close to Just Working as Windows ever does, and you'll be able to use as near to all of the installed RAM as makes no difference, if you don't go totally crazy with your video-card choice.
(It's now the end of January, 2008, and I'm giving this article a bit of a polish. DDR2 RAM is now so cheap that if your PC takes that kind of memory, you might as well buy 4Gb. You still won't get much more actual usable RAM that way, but two 2Gb modules may actually cost you less than two 1Gb and two 512Mb modules. So what the hell.)
If you insist on buying Vista version 1.0, it's still not a bad idea to stick with 3Gb of RAM and use the 32-bit installation option, because 64-bit Vista needs 64-bit drivers. 64-bit drivers are not necessarily fully cooked, or even available, for the hardware you want to use. Many 32-bit XP drivers work in 32-bit Vista (subject to the limitations I explained back in the first Ask Dan), but no 32-bit XP drivers work in 64-bit Vista.
(If you've got 4Gb of RAM, by the way, the Vista installer may not work anyway. You can work around that problem, if you have it, by pulling some of the RAM while you install Vista, then putting it back. Apparently Microsoft limit the maximum available memory in 32-bit Vista to 3120Mb anyway, though, so it's hardly worth the trouble of buying more.)
It won't be long before driver improvements and Service Packs make 64-bit Vista into a sensible proposition for people who want a PC with 4Gb or more RAM. Right now, though, there's very little point to it for normal users.
Yes, if you install 8Gb of RAM (the artificially-limited maximum for Vista Home Basic; Home Premium can take 16Gb) in 64-bit Vista, you'll be able to use at least 7Gb of that RAM, no matter what. But unless you're using 64-bit workstation/server applications (not just Photoshop), or multitasking a whole lot of big 32-bit apps, or running multiple virtual computers on your one physical one, then the extra memory will only give you a small performance gain.
Small enough that if someone steals half of your memory, you may well not notice.