I figured out how to get this card working in Mountain Lion:

Download the attached file (NVDANV50Hal.kext.zip) and extract it to S/L/E and replace the current one -- Repair Permissions, Reboot. You should have Native Resolution (1440x900) and full Acceleration in Mountain Lion for the Dell Vostro 1520 nVIDIA GeForce 9300m GS (256MB).

You do not need any other Enablers except GraphicsEnabler=Yes

I'd recommend copying it to the UniBeast/myHack USB Stick installer in S/L/E (you may need to "Show All Files") -- Replace the one in there, Repair Permissions.
You should have the Native Resolution now when you load the Mountain Lion installer (you can tell it's working then).

When the installation completes you can boot normally without KP and the need to use -x (the NVDANV50Hal.kext is what was causing all the KP problems)

DO NOT remove the GeForce* kexts or NVDANV* kexts - I read this in other posts and it will cause issues. Also, if you've been experimenting a lot already with different things I would probably recommend a fresh install anyhow (unless you can put all the original files back).

Again, this is for the Dell Vostro 1520 with the nVIDIA GeForce 9300m GS (256MB) graphics card. It may work in other laptops with this gfx card but I can't guarantee it. Make a backup of your original kext if you decide to try it...

Use MultiBeast to get Audio/Keyboard/Trackpad/Ethernet/Trim Support (SSD) working.

My Dell Vostro 1520:
-----------------------
128GB SSD
Blu-Ray RW
4GB RAM
nVIDIA GeForce 9300m GS
Core 2 DUO 2GHz
Dell Wireless 1510

GFX/Sound/WiFi/Ethernet all work in Mountain Lion. Card Reader does not (nothing new).

Please comment if it's working for you