onlineleben - 7:02 am on Jun 17, 2010 (gmt 0)
Sometimes it is not the hardware, but the software that is slowing down the system.
When you first installed your webserver and cart software, probably everything was running fine. Now over time as you got more and more customers and things slow down. Find out if there is a possibility to tune the software, clean up the database, get rid of abandoned shopping carts etc.
If nothing of the above helps, only then consider hardware alternatives