Hey,
I have both the x1900xt and 7300gt in my Mac Pro as well, and am currently running three 20" Apple Cinema Displays, and I can tell you that the 7300 is a nasty and slow card. The x1900xt despite being outdated a bit can still pull its weight around well, and should still be faster running both displays than splitting displays between cards. My 7300 chokes on the simplest of things and its really frustrating.
Putting a 20" or 23" in portrait position is awesome, I'm glad you like it
Reading blogs and webpages is much easier with all that extra screen space going up and down, instead of being wasted on the edges left and right of the actual content you see online.
I've been looking into getting a 30" and putting it in the center and putting my two 20's on the edges in portrait (A 20" in portrait is almost the exact height of a 30" in landscape, and I have them all set up on a tri-display stand with swivel mounts:
http://common.csnstores.com/common/p...G/EG1173_l.jpg I highly recommend this one!)