I have no idea, X is very very picky about how its set up, this is why I said follow the nvidia install directions exactly
one more thing you can try, reset it back to the generic nvidia driver "nv" and change resoltuion back to 1024x768 and make sure it works, then set it to "nvidia" and try it again, it might not be working at max resolution with the nvidia driver, you might have to go thru some trial and error
I belive X has a log file, check that and see whats in it, it will have clues as to whats happening, the monitor settings have be correct as well, close wont work sometimes