I wouldn't waste money buying the Phenom X4 9550 if you don't already have it, it's a tough call which would be better but they'll both struggle with the game. Due to the higher clock speed the Athlon would probably handle single player and small games better, but the larger more populated online maps(especially if you play 64player) will have that Athlon dropping single digit minimum frames. Due to how well threaded the game is, the Phenom should prevent that, but it's also got a really low clock speed.
Either processor is a bottleneck for that video card, you really should find out if any Athlon II or Phenom II processors are compatible with your motherboard and get a new X4. I would rather play the game on an Athlon II X4 @ 3.5Ghz paired with a Radeon 5670 than either choice you have.