I have been using FCP studio on my quad 2.8 mac pro with 2GB of ram and runs fine. I am editing 1080i but I just bough extra 8GB because I am having many software open at same time and FCP is crashing on me now.
But I used to use the same software on my quad G5 with 4.5 GB and they never crashed there, well, just once but after having the computer running on a project for 2 weeks with FCP, Lightwave, Cinema 4D, After Effects, Photoshop, Illustrator, safari, MS messenger, yahoo messenger and Toast Titanium and so on... at once. It crashed on me when I connected an extra monitor for the extended desktop.
Still... I left the computer frozen for a day because I knew it was running the renders in the background and it did!!!!
That was with panther, I do not know if leopard does the same.