Ah, our good friend Magento : (
I think your wide gutter is from the right column being TOO far to the right... see how there's no space between the right side of the right column and the right side of the page? At least not on my 17" monitor.
Did you just do that new upgrade they had? I had just done a bunch of work moving someone's main col to first-in-source and wrapping the sidebars, and now Magento's gone and done it in the upgrade. I don't understand the float: right AND the margin-right: -230px...
I see what Vineet is doing and it may work, if the right side is floated left then it will try to hug the wrapper. However, the wrapper should have been wide enough to fill the gap in the first place.
I also notice that, if you were to give all these major boxes ugly background colours, you'd see that col-main is actually sticking out of wrapper... maybe because wrapper was made a hair too small. If you keep a backup copy somewhere, you should be able to try out Vineet's code and see how that works for you (use the ugly background colours though so you can SEE the boxes you are playing with!!). If it breaks as you play with it, you can always go back to what you have now, because you will have a backup.