I've got a phenom X4 9550 on my old AM2 board, and I'm pretty happy with it, it blew my old X2 4200. They are both clocked at 2.2Ghz. In single threaded apps, the phenom is like 20% more or less in encoding at least. Here are my results as I posted earlier on another thread:
TMPGEnc 5000 frames Xvid (virtual dub as frameserver)
Athlon X2:
1 threads...133s
2 threads...71s
Phenom X4:
1 threads...108s
2 threads...55s
3 threads...50s
4 threads...50s
Phenom X4
2 instances of TMPGEnc + 2 instances of Virtual dub:
1 threads...207s
2 threads...106s
3 threads...79s
4 threads...59s
Just don't look at the "4 threads" option if you are going Phenom X3. I would recommend the X4 instead, mine got perfectly stable at 2.7Ghz.
In any case I think the Q6600 would be a much better buy, but in my case I would had to buy another motherboard and video card.