I was having problems with even 40-50 users at same time, 20members, and other were guests/spiders
From those 20members 10 of them were in chat, other surfing through forums or playing arcade games....shared server if you are lucky maybe can provide good "service" for something like this, but for short time (maybe)
BTW my forums are heavy on modifications, from chat to arcade games...
I moved to VPS server where I got 1.13 GHZ and 768 MB RAM ....
Now my forum is not walking, its
ps
my forums dont make any $, so this can be bad investment as vps goes from 20 to 70-80 $ depending where and what...