depends on the resolution. under 1900x1200 not much of a difference as theres enough bandwidth on the bus and at that resolution, it relies more on CPU. At 1900x1200, this is where the GPU comes into play - difference of a few frames - I'd say 10-15%. Above 1900x1200, it takes quite a hit, as it requires as much bandwidth as possible.
There wont be to large of a difference. I personally just ordered a ASUS P5Q Pro, so that I can reuse my memory and drop in a Q9550 around x-mas time with the ability to go crossfire if need be. I am not to worried about the performance difference. I think it will be fine. In most cases unnoticeable unless like others have said...at very high resolutions. I play at 1680x1050 and will never go higher than 1920x1200 if I so choose to upgrade my monitor. imo its not worth the price premium for the X48, but to each his own.
I could not wait for the DFI atx size board to come out so I went with the MSI.
It works great.
I stay away from Asus and by extension Asrock they have the worst RMA customer service. I took them 6 months to get a good board back in my hands, after 5 tries.
So I will not buy anything from Asus.