Some systems will handle two monitors, I don't have any specific
information. It's worth a try to re-install the GT520 but I don't think you
will be able to directly find and install drivers for the baulky card. It
may, however, be possible to download them from the NvIdia website and
install them this way. If you have a second monitor it would be interesting
to have both attached to really see what's happening.

How far do the others get ? If you hit [ESC] as they boot you should
get a screen with options for older kernels and also recovery mode.
Try either an older kernel or recovery more then purge the NVidia
drivers using Synaptic Package Manager and then let the system get the
latest driver automatically or starting the Additional Hardware wizard
directly.

Not sure how it would work with two cards but if you have only one
monitor be wary about switching between them with the system live as
this may cause damage to one or more card - hot-plugging monitors
isn't recommended

When booting the 3 ´recalcitrant´ distros in recovery mode , after
pressing ´resume normal ´ I get the normal display . However when again
booting in normal mode I don´t get any display,but in Ubuntu 12.04 coloured
areas/lines
But at least I can now try to find/install the appropriate Nvidia driver
possibly labelled ´nouveau´.
If the GT630 works with Mint 14 (12.10) it should also work with the 12.04
distros following ´normal boot´

This is the first ´quirky matter ´ with my 2 yrs old Intel i7 desktop
machine.

Booting Ubuntu12.04 in recovery mode ,I
managed to boot with the GT630 -4GB graphics card performing as intended.
Later I found in Systems >>>Additional drivers : nvidia accelerated driver
,version ´experimental 310´ when following installation I could resume
´normal booting´ . So this minor problem is now solved.
However I still don´t understand why ,prior to installing ´experimental
driver 310´ , the graphics card performed as intended when booting in
recovery mode .