with all the boards i've had (ic7, p4p800, 8rda+, nf7-s, an50r) i've never noticed an increase in stability or increase in overclocking of my video card by raising the vagp.
the only way i've gotten more is by volt modding the physical card.
i volt modded my 9700pro and have gone from 342/324 before mod to 428/352 after volt mods.
but YMMV...