Desktop Sharing application similar to webex or Dimdim

We are looking for a virtual meeting desktop sharing application similar to Webex. The web application should do following functionality. 1. Virtual web meetings by desktop sharing 2. Audio facility, Chat facility and video facility during conference meetings 3. Support upto 20 to 25 users. 4. Good GUI similar to Webex. Any creativity features are encouraged to share. We would like to go initially with less functionality. If the application is good enough to meet out requirements, once we get confidence with service provider, we will be giving more projects on the same. This project is a long term one. But initially we are going for webapp with less options to see the expertize level of developper. Other Details: 1. The webapp should be very light wieght application. So the users don't need to install any software. 2. Should work on Windows Vista and Windows 7. (If possible XP is also preferibly included) 3. Should check out possible options of working on Unix/Linux GUI OS 4. Should be user friendly and performance is a major concern. Service provider who had worked with or similar experience is preferrable. We are expecting any proof of previous work on desktop sharing applications. Thank you for spending time on this Project. Technology: Any (justification should be given for selected) Regards Gowtham.