It will likey be enough, but 30A@+12V is probably the mininum you'd want for dual 7800 GTX's.
I remember reading of a minimum of 34A@+12V recommended for dual 7800 GTX's in some article, but I don't remember where right now. These type of things are usually over stated, so 30 will likely be fine. But I wouldn't say that it's over kill by any means. Also, realize that this PSU is only rated at 460W continous even though it's called a 500W PSU.