In my home, I a bunch of computers.
ENVY - used by everyone, has the best CPU, RAM, video card, monitor, etc.
WRATH - alternate PC for when ENVY is busy. Usually running Age of Mythology for my 5yo.
PRIDE - sits OFF most of the time, but is available when there are suddenly a lot of people in the house
LUST - a tiny Sony Vaio laptop, usually OFF because it's way too slow running WinXP. Sometimes useful for MS Streets & Trips in the car.
HTPC1 - The media center PC. Primarily for DVD & music playback.
JAYBAZ_HOME - a Microsoft-owned laptop that I use to work from home.
SERVER - A, uhh, server running Windows Home Server Beta 2 (which is, like, totally awesome)
Only two of these PCs are the same model (WRATH and PRIDE). Each of the others is something different. Some can boot from USB, some can't. One is free of PS/2 slots, so I keep a spare USB keyboard in the closet just for that one.
Each one has exactly 1 hard drive in it, except for SERVER, which is filled to the gills. Each has exactly one optical drive - some are CD/R, some are DVD. Each desktop is a full-sized tower, which is way overkill for what I'm doing.
Keeping track of which PC can do what is a hassle. So is making sure they all have the latest BIOS. WRATH has trouble going in to standby on Vista, but the others are fine. Etc.
I'm thinking I'd like to move to a single PC platform (excluding laptops for now) for the whole home, to keep things simpler. I'm thinking:
Small is important to me, and shouldn't be hard:
Motherboard with the following features:
I want to be pretty low power, and very, very quiet. (Hey, while I'm at it, can it be cheap, too?)
For most roles that I described above, this PC is fine - add HD, RAM, and CPU and you're done.
For the main PC, I'd add a strong video card, and upgrade the CPU, HD speed, and RAM size. Some motherboards have onboard multi-channel audio, which might be nice. Alternatives include an internal sound card, and external (USB) sound "card", or a USB speaker set (e.g. these). The last one is nice because it leaves me with one less component.
One day I'm sure I'll want to do HD-DVD playback on the media center, at which point I'll upgrade the video, CPU, RAM, and HD on that box as well. But for now, the basic config is fine.
In the server role, I would take advantage of the incredible shrinking HDs (750G units are $300 these days), and either 1-2 USB enclosures, or a 4-drive enclosure if I really wanted a lot of space.
I'm hoping there's a barebones SFF PC that meets these requirements, and has good build quality, works out of the box with Windows XP and Vista, and is generally trouble-free.
We're approaching the end of days for the desktop PC. To be less sensational: the desktop case will continue to be important for both commodity PCs (they're cheap) and high-end PCs (room for high-end components) for a long, long time. However, unless you're playing the latest games (can't wait for HL2:EP2!), or processing lots of video, or something, you don't need the fastest that PCs have to offer.
For many years, the "good enough" PC was $2000 - that was the minimum you'd spend to get a PC worth spending money on. The PC that you really wanted was $5000.
Today, the "good enough" is more like $400-$500, and the PC you really want is $1000-$1500. It's hard to get a PC for more than $2000 without getting exotic.
Meanwhile, laptops have come way down, too. Instead of paying $1000 for a laptop that wasn't quite as good as your desktop, now the difference is only a couple hundred bucks. When you consider the savings in space, power, cooling, clutter, etc., the difference looks even smaller.
With that in mind, I think that I'm about 2-3 years away from a strict laptop-only diet. You might think that the media center PC would need to be a desktop to hold a tuner card (I don't watch TV) or lots of storage (I keep it on the server). You might think the server should be an exception, but remember that laptops come with built-in battery backup, and a compact keyboard/mouse/monitor that is good for a console you rarely touch. (Storage goes can go external, still.). Even the gaming PC could be an exception, but consider the Dell XPS M2010, with a Core Duo CPU, 20" widescreen LCD, a full-sized bluetooth keyboard, dual DVI out, 7.1 audio out, built-in subwoofer + 8 speakers, up to 4GB RAM, dual hard disks. Sure, it's $4000 when decked out, but that's still less than the $5000 "PC you really want" price of the early 90s. And the specs will continue to improve, and prices will continue to drop. In 2-3 years, it won't seem outrageous to get a laptop like that even for gaming, I suspect.
(Also, note that 3.5" hard disks will probably be phased out, even in desktops, in the next couple years. Consider how price, speed, power draw, noise, and heat are playing together here.)