speed problems on 7.0.2

My website www.sondeza.com is experiencing serious speed problems lately. Most of the time pages cannot even load unless the server is restarted. Sometimes users get an error message saying that a particular module does not exist even if that module exists.

This problem started after we got a lot of publicity, our network is now getting just over 14 000 unique visitors per day.

How many concurrent connections can version 7.0.2 take?

Do I need special expensive servers to cater for the 14 000 visitors? At the moment our processor is 4ghz with 8GB ram

My isp thinks that the issue could be caused by a dos attack(ip flooding). They've blocked the flooding ip's that that does not help the situation. Any ideas on what I need to do to speed up the website?

Rgds,

T