Will an i3-2100 bottleneck two 6870's in CrossFire?

In some games that can make use of more than 2 cores, (particularly at higher resolutions), yes a little bit. But theres big bottlenecks and small bottlenecks. Lots of factors that go into it. More information is needed. Like what games, what resolution? And... what kind of PSU is it? If its a crappy brand/model I'd consider replacing it.. wattage is one of many factors in determining what makes a quality PSU. This is important not so much for bottleneck issues, but the longevity of your components, as crappy PSUs have terrible voltage regulation and can burn out components.