I would say NForce 6800 Ultra, or GT, or GS, those are last cards from Nvidia that could be tweaked to Quadro, this particularly to Quadro 4000.
But problem is that this card cannot be purchase in stores (at least here in Cro), because they don't sell them any more.
So if you could purchase it, that would be great card.
I personally didn't try it, but from benchmarks and user reviews is great card.
As for other cards, I would also say Nvidia, some of 7xxx series, but I would say only 7600, 7800 and 7900.
Here are some charts for comparison of those cards:
http://www.nvision.pl/_plugins/content/content.php?content.1 04.10
and
http://www.au-ja.de/review-3d2006-b43.phtml
I would say that Nvidia 7800 GT or GTX would be the best choice considoring price and performance.
And one more thing, when you looking at card for ProE, you should look at card properties: GPU clockspeed and memory clockspeed!
If thous 2 speed are high card is faster, and have better performanse.
As for example:
Nvidia 7800 GTX 512 Mb GDR3
GPU clockspeed: 550 MHz
Memory clockspeed: 1700 MHz
VS
<br style="font-weight: bold; font-style: italic;">Nvidia 7900 GS 512 MbGDR3
GPU clockspeed: 450 MHz
Memory clockspeed: 1320 MHz
And benchmark results (cards that are marked blue):
http://www23.tomshardware.com/graphics.html?modelx=33&mo del1=523&model2=582&chart=212
Now you can see a difference between almost the same graphic cards, with the same amount of memory.
Hope it helps