I was wondering, if I were to buy a Mac Pro with one 7300GT graphics card installed, would I be able to buy 1, 2, or 3 more 7300GTs as an upgrade (and get a gaming performance boost)? I was wondering if they were just for added monitor support or if there was some performance enahancement. If the cards do work together like SLi, could I use a X1900XT and buy a 7300 or two (or 3) as an upgrade? Do you think 4 7300s would compare to a X1900?