CPU: You should definitely get a 64-bit processor, for forward compatibility reasons. You don't need to run a 64-bit OS; you can use a 32-bit one if you find you have any driver or software issues running in 64-bit mode.

I'm looking at one of the Intel quad-core CPUs for my next upgrade. I will probably get the Q6600.
CPU charts
The Intel ones prefixed with Q are the quad-cores.
AMD is coming out with their own quad-cores soon too, so you might want to wait and see if that affects pricing at all.

Motherboard: Not hugely important. Find one that has the right chipset for the CPU that you pick, and supports DDR2 or 3 (see next point).

Memory: DDR2 is the standard now, and probably will be for a while. DDR3 support is good for forward-compatibility; I think most motherboards support it.
2 gig is probably the sweet spot; I'll probably go for about 4. 8 is the maximum on most boards, but I've not seen any 2-gig chips yet (haven't looked very hard though).

Disks/RAID: I am no expert here; all that I can tell you is that striping increases performance and redundancy increases failure tolerance. RAID 5 or 0+1 is probably a good setup if you want both; this would depend on how many drives you choose to go with.
SATA is the ubiquitous standard now; SCSI is expensive, but better if you want ultimate performance.

Power: Get a unit from a well-known brand name, such as Antec or Hiper. I have a 730W unit, which is ample for almost any task.
Try and get a 'modular' unit if you can; these dramatically reduce cable clutter in your case.

Cooling: Stock fans are fine; it is more important to find a case with good airflow. Antec and Coolermaster are a couple of good brands. Lian Li have an excellent reputation, but are relatively pricey.

Sound card: No opinions. (I use onboard sound — don't laugh!)

Video card: Most have VGA+DVI outputs which will let you plug in two screens. If you want more then you will need to go for either an SLI setup (two identical cards linked together) or have a primary card plus a cheap expansion one.
VGA Charts

Case: see Cooling

Network card: consider this functionality when choosing the motherboard. Most have at least one NIC. Go for gigabit for forward compatibility, even if you don't have a gigabit LAN set up.


I hope that helps. I am fairly out of touch with the hardware 'scene', but that is all based on advice I got recently for my own planned upgrade.