If you don't need video and is looking for a similar hosted solution like WebEx, I say go with GoToWebinar. They can support larger groups, even up to 1,000.
If performance has been an issue in the past, maybe you should consider self-hosted products. Since its on-premise, you'll have more control over it. A good one is RHUB (http://www.rhubcom.com).
TurboMeeting for sure. I can use it for conferencing, webinar and remote desktop. The only downside is that there no VoIP right now. http://www.rhubcom.com
If performance has been an issue in the past, maybe you should consider self-hosted products. Since its on-premise, you'll have more control over it. A good one is RHUB (http://www.rhubcom.com).