I'm glad you're back up and running, LCL. But you made a huge leap in logic in declaring that all nVidia video cards are crap, or don't work with SH4.
It could be, that
A) The video card you got was defective(hey, it happens)
B) Your new power supply is not of sufficient quality or watts to handle all the hardware in your case
C) You had some sort of driver conflict that was wreaking havoc.
or even
D) You may not have had the new video card properly seated in the bus.
The only reason I bothered posting is that someone may find this post in a search, and maybe the possibilities I listed above might help them.
Anyway, I'm happy that you're back up & running. Sink'em all!