Allwin,

Yes we have developed , designed and are operating Dolphin sites with close to an average of 100K members.

You might be a bit hard pressed to find site owners or operators that are going to give out URLs to large population sites, as it tends to bring out the idiots of think they can hack or break into everything and it just slows sites down.

As well it makes it a target.

for which part of Dolphin is the most resource intensive, that would depend on the type of site you are running, see more if your site is going to be alot of Video Chat, then you will have more resource allocation to streams.

Out of the box, I found that Dolphin and Ray (more so Ray) had the biggest consumtion with out a specific use.

Your Question in regards to at what point do you need to look at upgrading or clustering, etc. This really depends on the site and what type of site and traffic it generates.

having 500,000 users reading a blog and looking at pictures is different then 500,000 users watching videos and Video Chat.

Hope this helped alittle

Regards,
CyberXing