Albeit brief, BareFeats.com has reviewed a dual Apple G5 and 2 Ghz Opteron.  And the winner is...

Now if I told you, you wouldn't read for yourself... :p

Some things to note on the review - done on 32 bit, not 64 at any point.  And the UT2003 test doesn't say which version.  Would have been better with a 64 bit Linux on there to do apples to apples like say SuSE or Gentoo... 

But there is mention of "round 2" of the review coming later - maybe they'll touch on these points there?