Well I'm hardly an expert on the matter, but I'll do my best to answer your questions.
<< So therefore, which card would you recommend for a 3D rendering station? Would the Radeon 8500 or Geforce2 Ultra (regular or modded) be better? Also, how good are the Diamond FireGL cards (Compaq pulls 32MB) compared to the R8500 or Geforce2 Ultra? >>
Probably the GF2 Ultra over the Radeon 8500. The R8500 has potential, and they also have the benefits of having obtained FireGL's driver team which I highly regard... but in the end I'd have to go with the GF2 Ultra because it's already proven itself to be a very capable card in that respect.
If you get the Ultra you may as well do the Mod, as I don't think it's supposed to be terribly hard to do... whether it will benefit you a lot I can't say.
<< how good are the Diamond FireGL cards (Compaq pulls 32MB) compared to the R8500 or Geforce2 Ultra? >>
Well, I'm no definitive answer on this one but personally I'd set the FireGL2 as a solid performer but below the GF2 U. It's not as good of an all around card as the Ultra but it's better optimized for professional level 3D rendering, and the drivers are definitely aimed directly at it and optimized for specific CAD apps. Maybe not quite as fast as the GF2 Ultra in CAD, but it's a proven "safe" choice.
I'm not sure on how the FireGL 1,3,4 compare though.
<< Are their other cards that I haven't listed or thought of that are in the $400 and less range that perform well in 3D rendering and can support dual monitors? >>
A TwinView supporting GF2 MX, or the Radeon 8500.
The MX is cheap, and despite it's position as a low end gamers card... it still possesses the GF2 core that's proven itself surprisingly adept in CAD apps.
Then there is the much more expensive, and unproven Radeon 8500. It's of the same lineage as the FireGL 8800, and should be capable of using the 8800 drivers.... but it hasnt yet proven itself as capable in CAD.
Honestly, I'd seriously consider the Radeon 8500 if your able to wait another month or so to get a better idea of how it acquits itself in CAD apps.
Otherwise it may not be best as it's simply unproven in that regard, and hence may be chancy.
<< And just a question that's always eluded me, how would a Quadro 64MB (non-DCC) compare to a Quadro2 32MB card? >>
In most situations the Quadro 2 32MB would outperform the 64MB Quadro.
What specific CAD/3D rendering applications will you be utilizing?
If I knew I might better be able to suggest an appropriate card.