CompUSA had a Phenom X4 on sale for ~$65 last week but it's back up to $90 after rebate now.
Right now the 450 is going for $70. If that's your price point, it's a good choice.
I have used a 455 and a 445. The 445 is in my daughters computer and she doesn't try to game with it.
The 455 was in one of my previous rigs and it did well with a GTX 560 (non ti) The only game I played with it was Skyrim and I was able to play smooth on the higher settings. (Not the highest settings though) The 455 also didn't seem to hold back the GTX 560 too much when it came to video rendering.
So guesstimating from that limited experience, I don't think it will bottleneck the 6670 card. If it does, I wouldn't expect it to be by much.