i would have to say that the t61p has the most powerful workstation card thats available in a 15.4". The 8510p is out of the question since it does not have a workstation gpu, unless you are talking about the 8510w which has a 570m(same card as the t61p, but cost more). The m4300 shouldn't even be considered because of its card. The m4300 comes with a 360m and at a higher price tag, while the thinkpad and the hp... you pay much less for alot more power.
I'm also in the market for a CAD/3d rendering notebook... I've been doing a whole lot of research.. and it seems that the thinkpad will be the one for me. You should do some research on notebookreviews.com they have alot of info on the t61p that you will find usefull... like the bad screen mfg...etc...
quadro fx 570m = 8600gt
quadro fx 360m = 8400gs
good luck with your search..