What would your main use be for it?
My main machine right now is the 17" i7 macbook pro and while it is adequate for all my usages, I think a desktop would fit more of my needs.
If I opt for the 3.2ghz mac pro with the ati 5870, I will sell my 17" (man I sure love it though..) and use the mac pro as my main computer. I have a netbook that I carry around back and to from campus, work, home. I found the netbook to be the more comfortable solution where it runs excel, microsoft project, word and everything I need more than adequately, plus I dont have to baby it around as well. Just pull the netbook out of my bag and type and do what I have to do and just throwing it in the bag when I leave is very nice but once I'm home the netbook is never touched, lol.
I also have the habit of leaving my 17" hooked up to my nec monitor and 95% of the time used in desktop mode. I feel as if it is a waste using a laptop as a desktop when I can get something so much more powerful for a little bit more in the cost.
But one of the main contributing factor for aiming towards a Mac Pro is storage. I need a lot of storage. I keep having to erase all my contents to make room and I'm already daisy chaining 3 external hdd and its getting very cluttered as well as the 2nd and 3rd drive being slow when extracting or moving files around.
I would love to just stick 2tb hdds in the mac pro and just be done with it. I was thinking about the 2.8GHz model but might as well get the 3.2GHz as its default clock would be very nice and turbo boosting up to 3.4GHz would just give me that wow factor!
Only gripe is that I'm in love with the 17" macbook pro as I got a near perfect one. The screen has no stuck/dead pixels or any type of backlight bleeding. The speakers are very uniform and loud and clear without any static either...
But those shouldnt be the only reasons for me to keep the 17", another thing is that since it is a laptop, I just like the idea of packing it up and just taking it with me on rare occasions (but I could live without it).