just seen this thread

its showing the same problems as I had with my underpowered card (but probably not crashing like mine). It'll be to do with the 12 volt rail output on your power unit. Many power units now split your 12 volt supply over 2 or more rails resulting in insufficient ampage driving your peripherals.
I'm guessing your new card uses an amount of power that is borderline to what your rail can supply.
You should look around for a larger power supply preferably with a single 12volt rail. They have a few different ratings on the side/underneath of the power units, showing 3.3 volt, 5 volt and 12 volt. That section will tell you if it is a single or multiple rails and what ampage each rail can deliver.
If the GFX card is sharing a rail with another peripheral then that might also be causing the problem. You might find disconnecting other things like the DVD drive or an external hard drive etc might give you a boost but it'll only be temporary till you plug something else into the system

Edit: just for reference the wattage of the PSU doesnt necessarily mean a fix. I had a 550w unit with two rails that was casuing a problem so I popped up the road to a computer shop and ended up with a 500w unit on a single rail and it worked fine.

Good look
Si