Ok, finally had time to do the swap. Getting the old card out took some effort - the release lever at the back worked fine, but the "front end" of the card was well & truly settled in place at the bottom of the slot & took some pulling to get it out!
The GT220 uses the same Nvidia Driver as the G310 under Win 10, and the latest version had auto installed when we upgraded just recently to Win 10, so nothing to do there :-)
The new card is the Gigabyte version with a nice set of cooling fins & a big Fan - the 580 is now almost totally silent on start up, & we still haven't really managed to get the fan "excited", though being mainly a work Computer, You Tube Videos are probably the most arduous work it has on a daily basis. But much better than the "stock" card that came with the Computer, which would sound like it was a propeller plane about to take off if we used "Bubbles" as a screen saver! Again, not a sound from the new Guy :-)
So all in all very worthwhile as an upgrade!
Now we have the Graphics Card & Seagate SSHD in place, the Computer feels like new. Would be almost tempted to upgrade from the i3 to a compatible i5 processor, but reading what is necessary to do this, I feel it might be beyond my computer part swapping skills :-)
Thank you Hema for all your support & advice throughout this - I doubt we'd have got here without it!
The new card is working great! Very Quiet, system loads faster, no slow down if many windows open. A very positive upgrade