Yeah, as long as its got the right plug you're fine. I use a 24" Dell on my Macbook.
With the Mac Pro you will be fine. I believe all of their cards have dual dvi.
Generally speaking. But the OP asked about a 30" screen, too. A 30" screen with dual-dvi output will only work if you have a dual-dvi video card on your mac pro.
Then again, I guess that is just another case of "having the right plug".