don't know why my forum goes so slow with 2.4 dedicated

I have a 2.4 dedicated server and 2GB ram

and have installed the MMcache and change MySQL's my.cnf to my-large.cnf

BUT when prople goes to 180 online, still slow.

WHY WHY WHY ?

What forum are you using? invision, phpbb, etc...

phpBB

I had seen sites run 20000 phpbb sites and with peak 500 on but no lag or anything. Do you have any extra code you add? Stats? I have found out that the stats sites will lag up your page.

yes , I have tried to remove those stats, but doesn't change any thing!!

Is there any badly written modules integrated in your phpBB?

I would recommend you to use Invision for anything huge :D

I have a server with P4 2,4Ghz and 1,5GB RAM (with mmcache) and currently have only one phpBB forum there with 200MB dbase and 60 people at peak hours.

The first thing I noticed is: running phpBB is very RAM consuming consuming. 1GB RAM for my site was not enough (and I only have 1/3 as many users as you). The problem were many big httpd processes. CPU power wasn't and still isn't a problem. Are you sure that you don't run out of memory at times of big load?? Maybe the server is swapping to the disk?
I had seen sites run 20000 phpbb sites and with peak 500 on but no lag or anything
There's a nice phpBB forum with 1500+ users online but they split the load between 4 or 5 different servers. I think that a server with 500 users online would need to have a huge RAM and CPU power or it's a forum with tiny database - number of users online is one indicator but the database size is also very important.

Make sure to install the extreme style mod, much better templating engine