One tip: nVidia 7000 series graphics card for OpenGL, and hold off Vista for the moment. The Radeons suck at OpenGL. Hardware is good, drivers are bad.
While graphics cards don't do anything for render speed (that's CPU only), good OpenGL is a must for the preview screen in Poser, Vue, and many other 3D graphics apps.

Around $900? My recommendation would be:
Intel E6400 ($180) or E6600 ($210) CPU
MSI Neo-2 FI P965 mainboard ($100)
7900 GT graphics card ($220)
Seagate 7200.10 320 GB drive ($80)
Antec case with 550 W PSU ($120)
4 GB (2x2 GB) DDR667 ($220)
DVD +/- RW ($35)
WinXP Pro 64 bit (works well with both graphics card and mainboard)

You should be able to get these components at TigerDirect or Newegg for about these prices.

The MSI mainboard has 5 SATA II connectors, FireWire, a plethora of USB 2.0 ports, Gigabit LAN, 1 PCIe x16 for the graphics card, 1 PCEe x4, 1 PCIe x2, and 2 standard PCI slots. Supports up to 8 GB of DDR2. And MSI delivers rock solid mainboards - Asus is a bit of "hit and miss" nowadays (a shame, actually. Asus used to be a top level brand).
The Intel 965 chipset has the fastest RAID solution currently on the market, and a very nifty feature called Matrix RAID: you can create different types of RAID arrays without installing an insane amount of disks, just by using "chunks" of the disks. So you can have a RAID 1 plus a RAID 0 using just two disks, all other RAID solutions would require four disks.

The system outlined above is faster than any AMD based system on the market. Not by much, agreed, the 5600 and 6000 come close. 

Assembling a PC out of the separate pieces isn't that hard. If you can read manuals and if you know the business end of a screwdriver from the other, you can do it. Just don't hurry, and make sure ypu've got the front panel connectors right before you apply power to the system.

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

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