Not necesarily. AGP aperature is the amount of system memory the video card can take control of if needed. Since you have a 258 mb card, I doubt that it will ever need to borrow system memory, at least not for todays games.
I have a 128 mb card and noticed a small increase in the speed battlefield maps load when raising the aperature from 64mb to 128mb. Just play around with it and see if anything works better than something else.