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Greetings Andy:
The "G" actually refers to a unit of gravity (which can of course be real or apparent--the former from a gravity field and the latter from a force such as centrifugal force).
(As a side note, you may want to check out Tim Gregson's FRATE2K in our downloads section. It actually displays your frame rate average rather than that fluctuating thing that the stock frame rate counter displays.)
Your observation of a better frame rate using a higher resolution is really video card, driver, and system dependant. Not all users (in fact, the opposite is more common) will experience a higher frame rates at higher resolutions.
Here's my guess at why you're seeing what you're seeing:
Your video card may not display as many colors at 1024x768 as compared to 800x600 (e.g. the higher resolution has a lower color depth). Of course I hope I'm wrong and you're just very lucky! But if this is the case, try running 800x600 at a lower color depth. With all things being equal, you might see even faster frame rates! And as well all know, when it comes to sims, high frame rates are a good thing!
Best!
Ben