Other advice for the GTX 295 going into a blank screen, or indeed any other nVidia GPU, it goes like this
"Try all available display ports"
When the driver activates, it might only activate one of the display ports. I have an EN210 and out of the box it will only work using the DVI port, and NOT the VGA... I was originally jacked into the VGA port and was confused by the black screen bootups, turned out the driver only activated one of the outputs.
Same with the GTX 295... if yours is anything like mine (i suspect they are all the same) it will have 2xDVI and 1xHDMI
I have read that when the card is set into Non SLI mode, only one of the PCBs/GPUs will activate a display. This apparently has a caviat that the card will want to use the HDMI output first. In Windows the driver i think takes care of it and gives you the DVI back. Is OSX, i am not sure if that is the case.... however I got at least one of the GPUs up and running out of the box using the following method.
install Lion 10.7.0 and update to 10.7.3 via the combi download.
install the driver from here http://www.nvidia.co...f01-driver.html
boot up using graphics enabler = yes and did not use anything else graphics related.
When my display is plugged into the DVI ports (either of them) I get a blank display... all looks bad... oh dear. But jacking in the HDMI and bingo, OSX is in fact running and at least one of the GPUs is active.
For me this worked perfectly, full acceleration and resolution. So when you get a blank screen of death on the 295, check first if the hard drive is still active, i found that despite the display looking dead, the rest of the computer was very alive. Then try jacking into the HDMI port rather than one of the two DVI.
Only thing i havent looked at is if both PCBs are active as in my system report i have two display adaptors, one simply "nVidia Display adaptor" and the other "nVidia GTX 295"
This was achieved about 2 hours ago so more testing is required
While i understand that most with a GTX295 want to use it for OpenCL related bits and bobs, and the occasional game etc, i think for most who are just interested in having their hackintosh working and usable, spending days trying to get both GPUs working should only be done if you are an absolute completest or if getting them up is trivial (it might be, i haven't tried it yet) The 295 is a nice graphics card even when only running half capacity though