The specs in the first post were way out of date, the second setup you listed vince3 looked better.

A good hardware site I'd recommend is http://www.anandtech.com/

A number of their performance reviews include benchmarks for 3D Rendering Applications (no Poser of course, but at least will give an idea on rendering).

An example from a recent CPU review: http://www.anandtech.com/cpuchipsets/intel/showdoc.aspx?i=3012&p=7

For purchasing components, check out http://www.newegg.com/ as you shop around. Great prices and selection and they have user reviews, quite a few for the more popular components.

No soldering required to assemble your own machine, but you need to make sure you get all the right components for the right slots. :) The quality of instructions that come with motherboards and other components vary considerably, it's a good idea to look for some guides on the web on how to assemble a PC. I think for a first timer installing the RAM (installing RAM requires more force than you might expect), and the CPU are the most nerve wracking, I still get a little nervous when installing the CPU/heatsink/fan and I've assembled my last four PCs myself!

As to ATI vs NVidia, I have heard anecdotal reports that currently ATI's OpenGL support is better than NVidia, which would be a reversal from a couple years ago. Try checking out specific reviews for cards from both manufacturers you are considering and see if you can find any specifics on OpenGL performance.