I've been reading the NVidia guides which to a Linux newbie like me are more than somewhat intimidating, but I infer from PantherX's comment earlier that Mint contains all the necessary source files etc and that the NVidia installer does all the hard work.
I've obtained a copy of the NV 319.49 drivers that ChristianVirtual seems happy with, so I'll use them.
I've outlined what I think I need to do, some reassurance before I start would be appreciated! (No great rush- I probably won't have time to do it until the weekend at the earliest, and doing something unfamiliar like this in a hurry is usually a good way to ensure that whatever can go wrong, will)
1) Set client to 'finish', 'pause-on-start', when it stops set 'cpu' to 3.
2) Shut down, install the hardware, connect spare monitor to GPU and reboot. (I intend to use the motherboard graphics for the primary display rather than the GPU, but I'll sort that out when it's all working)
3) Now a couple of bits I'm not sure about- do I need to stop the X server (init 3 ?) and do I need to do anything about the /etc/inittab file to prevent it restarting on a reboot until I want it to? Because I can't find a file called inittab, in /etc or anywhere else! (Although there is an 'inittab.5.gz' archive in another folder)
4) Run the installer and do as it tells me.
5) Sort out any bits (like primary displays) to my satisfaction and start FAHControl, which I assume should have detected the GPU automatically and set its defaults appropriately?
6) If all looks OK, right-click and select fold, and watch the points pile up

A general question- how (or if) does Mint's Driver Manager come into it? I ran it but (not surprisingly) it came up with no proprietary drivers. Is that only of use after I've installed the drivers as above?
Thanks in advance.