I'm pretty far behind on these issues but here are some thoughts -
Choose your CPU first, as it pretty much predetermines many of the other options you can make. Here
is the chart you knew existed but tried to ignore. I'd go with a Athlon-64 if cost was a major issue. Otherwise I'd go for a dual core one (e.g. Athlon 64 X2 3800+) - a second CPU's effect on performance is quite negligible but (with a properly built and configured OS) it does wonders to a computer responsiveness.
MoBo - an often neglected item, but one that has the largest impact on the character of the computer. There are many players in this field, so it's hard to make a specific recommendation, but I found Asus/Abit/MSI to be decent so far. Skim its data sheet before you buy it to make sure it supports the features you're interested in (CPU type, memory configuration, number of SATA channels, etc.).
Memory - You'll be using DDR whatever you choose above (like with CPUs, not having a fast one is no longer an option). There are several flavours of DDR, with various speeds and bus widths, but the choice of CPU/MoBo will limit your options here. Choose dual channel if available. DDR2 is the current bleeding edge. As for amount of memory, this is easily upgradable, so saving here until the mandatory migration to Vista is an option. I have a gig and a quarter, and I never seem to be getting around to using that quarter :-).
Hard drive - It is a sad time to be a hard drive owner. Even if you get a well made one and keep it cool and ventilated, it will die. Soon. Usually without a warning, never after a backup. So I'd recommend going RAID. It doesn't have to be a full hardware SCSI monstrosity, just buying two drives instead of one and telling Windows to use mirroring should do wonders for your sleep quality.
Display adapter - 3D acceleration starts being used in a lot of little places besides games. Its use as the engine behind GDI has been delayed a while, but it will get there. Get a modestly capable card (just pick one of the Radeon/Geforce variety).