an AIW card is never gonna be the best card for gaming since its designed for a multimedia system but 4790 is ok. I'm not sure what a 7900gt would get but it would most likely be higher by a little bit.
it doesn't actually have 48 pipelines, it's only 16 pipelines. it has 48 pixel shaders. it's kinda complicated, but basically, there's 3 pixel shaders per pipeline in the X1000 series cards http://www.answers.com/topic/x1900