I second Mike Rinehart's suggestion of the Blue Yeti. Although I haven't used one personally, it is one of the most popular budget mics on the market. Its switchable pickup pattern means you can set it to capture sound from all directions equally. Interestingly, it actually has three condenser capsules, which allows it to be used in a wide range of applications, and its integrated stand would work very well on a conference table.
However, it sounds like this conference room would not have everyone equidistant from a central microphone, in which case the people that are farthest from the mic might still be quieter enough to make them hard to hear. I'm sure the balance would be much better than the MacBook mics, but still probably not the best. If this is a problem, you might want to go for two or more identical mics, placed such that everyone is nearly equidistant from a mic. I'm not sure how you would be able to configure two separate mic units for a conference call, but I'm pretty sure it can be done.
Of course, another option would be to go with one or more microphones designed for conference calls, which would likely have a similar omnidirectional pickup pattern. Although from a quick search, it looks like a lot of them are equal or greater than the Yeti in price, and since the Yeti is a more well-known and widely acclaimed product, you might prefer going with it.