It's of some comfort, vnzjunk, to know I'm not alone on this. I've got to a solution - found help on the XBMC forum to calibrate within XBMC, and now using a setting of 1280 x 720 in linux mint. To be honest, I'm not sure how I got to that point, but I used nvidia control, which I installed. There is an ongoing problem, though, in that if I try playing HD videos e.g from Vimeo website, they play, then randomly my system seizes up, requiring a hard reboot. I've had several 'false starts' with linux distros over the years, but never got passed the difficulties and frustration of having to work from the command prompt. I've got linux mint 10 on another computer, which seems to work quite well - and hasn't crashed like mint 12. It feels to me, that the newer distros are almost there as viable MS alternatives for end users, but still not user friendly enough, if even seemingly simple problems arise.
Must admit, it's been disappointing that no helpful suggestions have been forthcoming on this forum, when I assume this forum is the best one for all things linux mint.
Now, if only I can sort out why my system freezes and how to access a Windows shared folder using samba or otherwise ...
Good luck with your quest