Okay, my 2 cents.

AMD versus Intel: AMD used to deliver much more bang for the buck. That has changed last summer when Intel released its Core 2 Duo line. If you go for new hardware, I HIGHLY recommend an Intel Core 2 Duo. Either an E6400 or E6600, they are FAST and fairly cheap.

Mainboard: go for a GOOD brand. Asus, Abit, Intel (highly recommended!) MSI, Gigabyte, Chaintech. 
The best mainboards have passive southbridge cooling, which means there is no fan on the mainboard itself. Southbridge fans have the nasty tendency to stop working after a year or two, and an overheated southbridge means the end of your mainboard.

I can highly recommend the Intel BadAxe 2 mainboard. It's not cheap, but it fully supports the Intel Core 2 Duo chips, including the quad core QX6700, and it can hold at least 8 GB of RAM. Probably (I'd have to find out) even 16 GB.

Graphics: go nVidia. ATI makes excellent hardware, but the drivers are crap. Only DirectX tends to work well. The OpenGL support is lousy in my experience, except when you go for a FireGL card. The FireGLs are professional graphics card with certified drivers, comparable with the nVidia Quadro series. Very, very expensive, usually slower than their consumer counterparts. Not recommended, unless you use 3DS Max, Maya, Houdini or another high end 3D package with dedicated drivers.
Poser does not have dedicated drivers for professional 3D graphics cards. Neither do Vue, Bryce and DAZ|Studio. So a professional graphics card would be a waste of money.

A fairly fast nVidia graphics card is your best bet. 7600GT or 7900 GS/GT will do the job, not overly expensive, and it'll be fast enough for a couple of years to come.
Go PCI Express. Mainboards that support AGP are hard to come by, and if you can find one, it's probably relatively expensive.

RAM: Go DDR2. By now, DDR2-533 RAM is as cheap or cheaper than DDR400. Besides, DDR2 comes in larger modules, I've already seen DDR2-533 sticks of 4 GB each! Most mainboards have 4 memory slots, so you could upgrade your system to 16 GB using DDR2-533. With DDR400, the largest sticks are 1 GB, so you'd be stuck to a maximum of 4 GB.

DDR2-667 or DDR2-800 is faster than DDR2-533. The influence on system performance is not much, however, in a test using Vue 6 Infinite DDR2-667 performed only 1% better than DDR2-533 (while it SHOULD be 25% faster). DDR2-667 is much more expensive than DDR2-533. A waste of money.

Be prepared for 64 bit. I'm running WinXP 64bit on my Athlon64x2 4400+, and it's fast, reliable, and I can build and render scenes that were completely impossible under WinXP 32 bit. 
I don't like Vista, I tried Vista RC1, and the visual goodies made it look like a Mac. So I couldn't find anything. 

By the way, my Athlon64x2 only has SATA drives. WinXP 64bit installed without a hitch, no drivers needed for SATA.

Case: go for a good midtower or full tower. Antec, Chieftec, Coolermaster are good brands.
Power supply: it's all been said. Get lots of power, at least 500W, and get a GOOD quality PSU. The stability of the power output is more important than the raw amount of watts, I once exchanged a 400W Q-tec PSU (cheap) for a 300W Papst (good), and the system immediately ran more stable.

Fans: go for 120 mm fans wherever possible. Those run more silent than the older 80 mm fans, and their air displacement is significantly more. Whether you can use 120 mm case fans depends on the case.

Putting a desktop PC together from quality components is not that difficult, they usually come with a good manual. But if you don't feel up to building it yourself, most shops that sell the parts will assemble the machine for you for about $50-$100. Including warranties.

If you can spare the money for a monitor, I can highly recommend Samsung Syncmaster 19" displays. I've connected two Samsung Syncmaster 913v screens to my main Poser workstation, and it works like a dream! One screen for the scene, one screen for all the tools and palettes.

My recommendations in short:

Intel BadAxe 2 mainboard
Intel E6400 or E6600 CPU
2 x 2 GB DDR2-533 RAM
nVidia 7900 GT/GS graphics card (preferably GT, significantly faster)
Good quality midtower or fulltower case with 120 mm fan openings
Good quality PSU with at least 500 W output, and enough connectors to power just about anything.
Seagate 7200.10 SATA drives. Preferably 2 drives.
DVD burner. Go for a dual layer +/- RW drive, they're less than $50 these days. 
Floppy drive.
Samsung Syncmaster 19" flatscreen(s).
Windows XP 64 bit.

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

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