To all that have problems with a Dell D630, I report mine is working normally. I'm using :
FreeBSD gusiport 8.0-STABLE FreeBSD 8.0-STABLE #3: Wed Dec 9 09:57:39 CET 2009 gus@gusiport:/usr/obj/usr/src/sys/CUSTOM amd64
without problems so far. I used the driver with 8.0/RELEASE and worked fine too.
I recompiled all the ports from scratch when moving from i386 to AMD64. Performance is good (don't if better than i386/195.22, didn't try any benchmark). No black windows with loose-binding (with i386 I had those when opening three or four gnome-terminals without tabs).
Moreover I don't get any freeze when logging out. I tried going single-user (with shutdown) and returning to graphic mode, and it worked nice too.
Right now I retrieved my old xorg.conf from my last i386 backup. Xorg works fine with both my old xorg.conf and a new brand xorg.conf generated with nvidia-xconfig.
If you want I can provide my loader.conf and sysctl.conf. I don't know whether they may help or not.