The Mac Pro has three 'single-width' slots, and one 'double-width' slot. The Radeon X1900XT and the Quadro 4500 are both 'double-width' cards. That means that you could fit two of those models, and have one spare slot. (As one card would use the double-width slot, and the second card would use two of the remaining three slots.)
You could also go for four GeForce 7600GTs, which can each drive one 30" display, plus one up-to-23" display (or, more technically accurately, one display with a resolution greater than 1920x1200, and one display with a resolution of 1920x1200 or less.) So you could have four 30"ers and four 23"ers.
Or, you could go for two Quadro 4500s and one 7600GT, for FIVE 30" displays, plus one 23" display. (Two 30"ers on each Quadro, plus one 30"er and one 23"er on the 7600.) Yes, you could theoretically have two X1900XTs and one 7600GT, instead of two Quadros, but since the Quadro and the 7600GT are both made by nVidia, I would assume fewer compatibility issues.