Like 0Vman, I am also a member of Legends of America , and I've had good experiences there as well. LoA has actually had a fair many more than 150 topics, but the admins like to delete old topics from time to time . Suffice to say, our boards are active, we have regular guild tournaments and contests that reward peoples differing skills, and if you are looking for a specific card, many of LoA's members will help you watch the market for good deals.
That said, I would also like to nominate the Unicorn Society for your consideration: http://www.urban-rivals.com/guild/?id_guild=90240
They aren't particularly large, and I don't even know anyone in it (other than some brief message exchanges with the guild founder, who I hope doesn't mind me promoting his guild ), but they have the flat out best guild concept/ description of any I've seen. Go Fighting Unicorns!
Good luck finding a guild, there are a lot of good ones out there .