Mainboard: not too many bells and whistles. The more stuff on the mainboard, the more potential conflicts.
For Intel CPUs I'd recommend mainboards based on an Intel chipset. P35 is the most common chipset right now, and it works just fine.
SATA-II drives: Vista installs out of the box. XP needs drivers on install. That can be a pain when the new machine has no floppy drive.
Luckily, you can incorporate those drivers into an XP install CD using nLite (freeware). It's easy to create a custom XP install CD that way.
Brands of mainboard - I've had good experiences in the past with MSI and Asus, but my latest Asus mainboard had a lot of trouble. First one broke in a couple of month, got another one under warranty that didn't work at all, the third one (I still have it), works, but some of the goodies have problems.
I'd stick with MSI, Gigabyte, Abit or Intel.

If you plan to use multiple monitors, you cannot use SLI (two nVidia graphics cards working in tandem) or Crossfire (two ATI graphics cards working in tandem). Stick to a single graphics card. As of now, an nVidia 8800 GT is by far the best value for money. Don't fall for an ATI, their OpenGL support is too flaky.

Hard drives: Samsung is good. And very affordable. Maybe not the fastest (WD Raptors are FAST - but very expensive), but great mainstream drives. And they're silent.
Seagate Barracuda is also good.

For speed, I'd definitely recommend installing more than one drive. By distributing OS, applications and data across multiple drives you can gain quite a bit of performance.

No use going for expensive fast memory. You'll get a performance increase of 1-2%, which is not worth it. DDR3 is too expensive, right now DDR2-800 is the best value for money.
And get lots of RAM. I can definitely recommend installing 8 GB.

The pen is mightier than the sword. But if you literally want to have some impact, use a typewriter

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