Numbers? check the discussion group,
http://discussions.apple.com/forum.jspa?forumID=1189
it has some serious limitations as a replacement, let alone for size and number of cells. whether that is due to being 1.0 or something else, I don't see it as a replacement or challenge or whether it will even run or import as is, but you can demo the full version for 30 days.
I don't find much of any difference in stability or reliability between OS X and Vista, but I expect Leo (10.5) will up the ante.
The Mac Pro is ideal where you need more than 4GB RAM (which many would consider a good base, with 2-3GB as minimum); multiple hard drives is nice. You can install and boot from any of your disk drives as you wish, a real plus. And graphics is actually weak. The OEM card is an the lowest rung, and X1900 is a year old and mid-range only.
Memory costs more, due to using ECC FBDIMMs.
A quad-core anything would do nicely, and running XP you can tweak away to quite an extent.