without know how much ram is in the system and what operating system your running then we can't really say what's best for your situation, newer operating systems need more usable ram to work without issues, those settings are used to set aside x amount of your ram to be used only by your graphics so might effect how the os runs because that means x amount of ram is permanently unavailable to it.
agp is used when you have a dedicated graphics card installed and vga is used when your using onboard graphics.
so really if you have a lot of system ram and can spare losing the max amounts without it slowing things down then set them to max, probably the thing to do though is experiment and set them both to max use the pc and see how it goes, set them back to default or lower settings and test again then decide which settings give you the best user experience when using your pc and stick to that setting.
should add if your using a dedicated graphics card then you can just disable the vga setting and if using the onboard then set the agp to lowest setting, only one of the settings are actually used depending on what graphics your using anyway.
Edited by terry1966, 19 March 2016 - 02:40 AM.