I would also reccommend downloading the demo to try it out. There was a thread here recently about MX 2004 on panther, and the performance is completely unacceptable. In my own experience with the demo, having more than 4 or 5 files open at once in Dreamweaver MX 2004 slowed to program to down to an unusable crawl.
My advise would be to stay with MX (v6). But at the very least, try the demos to be sure you get at least the performance from MX 2004 that MX is currently giving you (many are not).