Exhaust is good, but no coolers have exhaust for the x1800 or x1900 cards so forget about exhaust coolers. Whether you choose the Accelero X2 or the Zalman vf700cu you are going to have exhaust in your system. I recommend the X2 for the same reason I posted in your other thread plus it is a little quieter than the Zalman.
Also, another problem with the Zalman is that it does not connect to the card's fan socket and there have been issues where the card will not boot unless a fan is plugged into the card. This could render your purchase of the Zalman completely useless.
And as far a temperatue dissipation goes, well both the Zalman and the Accelero X2 are the same, no major difference, but IMO, the Accelero is better because it droped the temps of ym card by 15c idle and 20c under load. I have yet to see results like that for the flashy Zalman.
Check out these two pages for a brief comparison: