In some situations its definitely a bottleneck. I've been benching my x3 720 + 5870 for forum wars. What I've noticed is that in Vantage overclocking the gpu has little effect on the score, while oc'ing the cpu increases both the cpu and gpu scores by a similar amount. At least in Vantage the x3 720 bottlenecks a 5870 by quite a bit.
Sadly, the games I'm looking forward to the most this year appear to have minimal requirements.
P4 + 6800gt
Dragon Age 2:
Blizz games always have low system requirements