#1
I'm kinda thinkin that instead of buying something new maybe I should concentrate on making what I've got better. I've got a late 90s Sheraton and whilst I don't overly want to change the look or sound of it I would be interested in making it sound/play/feel/perform better. But what to do?

The tunings (especially G string) not fantastic - New machine head? (if so what? grovers???)

Surely I can make it sound better??? New Pickups?d (which? - to maintain similar sound - just nicer)

Bridge????

Electrics???

I'm totally lost as to how to 'improve' it.....
#2
you mentioned grover tuners, and thats a really good place to start. I've got mini locking rotomatics on my strat, and a full size set (whose model name I can't remember) on my breedlove, and both sets work really well for me.

I'm not really sure of the quality of the bridge on a late 90s sheraton, but there are certainly aftermarket bridges you can upgrade to if you want. like I said, though, I'm not sure of the quality of yours.. if it's fairly good, then I wouldn't make it a priority to upgrade

electronics could be upgraded along with the pickups, but I'm not sure what to suggest because you say you want the same sound, just improved..
if you like the tone of it now, keep the pickups and electronics stock. otherwise, what sort of sound are you aiming for?

without knowing the exact specs of your guitar, it's a little tough for me to say "upgrade to'x' pickups".

but it sounds like you just want the overall quality and feel to be improved, so.. do you know how to set up a guitar?
I'm thinking after changing the tuners and possibly the nut, you might be happy with it if it just gets set up properly..