2.4v is WAY TOO MUCH. i wouldn't go higher than 2.2v or so.

the 7900 isn't memory bandwidth starved anyway, so raising memory that high would net you hardly any improvement.

Also, if you raise vram to 2.2v or beyond, you should put a heatsink by the ic next to the mod, because it will get very hot and cause instability.

as for core, what temps are you getting under load? what are you using to cool it? if you have good cooling, you can give 1.45v or 1.5v a go.