mac news sites that compared raw graphics performance saw no advantage to the 4500 over x1900. i have not tested them.
there is a big difference between the 7300 and the x1900- the x1900 is much better. ( i just upgraded a theatre show from a 7300 to an x1900 - with a vast improvement )
just because it says the newest triple head will do 75hz at 3072x768- does not mean it will work. you will have to test it for yourself.
and i would not ever recommend that anyone does this on pre-intel machines - they are just not powerful enough.
there is also the vqs-01 which i have been told has been used successfully on shows in london
http://www.datapath.co.uk/vqs01.htm
this has 4 outputs.