"Beside missing VGA port, the memory of my GT 420 read from Device Manager is not 2 GB. It's a very strange number."
Where are you seeing that in Device Manager ? - I don't see any figure for the amount of video memory in there.
It's in System Information.
E.g. Click on the Windows 7 icon bottom left of the desktop screen, in the Start Search box type: msinfo32 , press Enter .
On the left side, open up Components, select Display
Adapter ram line.