We just upgraded both the Cloverton & Harperton Mac Pro to ATI 4870’s. Though the 4870 is listed as only working with 2008 and 2009 Mac Pros, it worked in our Cloverton Mac Pro, so far… We were having rendering issues with the ATI Radeon HD 2600 with some of the more intensive effects in HD.
You’ll need to trash your FCP prefs so it will rebuild the RT Profile.