The only way youll know if your card supports it is to see the manual, or the exact spec on their website.
The question of stability again is impossible to know without trial and error and would vary from spec to spec of the PC not just the video card.
Even more issues arise with different applications using the card, and software associated with (in particular) 3D intensive apps. For example enabling SBA for one app may enhance one and not another....then again you may see little difference. Modern video cards these days with 32 Mb or more have little problems coping with most of the graphical processing, without having to touch the RAM. Also your motherboard has to support some of the agp attributes anyway.., for your card to use them. My guess is that these attributes wont make an awful lot of difference in terms of performance on your system, though may well effect stability.
If you really want to test it, i would reccommend 3D Mark 2001, which you can do control tests under different circumstances, and get a numerical result for comparison..