Before spending money on a new server, have you done any performance monitoring? Have you tried to find out why performance is becoming slower? Have you tried reconfiguring resources to improve performance? If you have a performance issue then throwing money at it will not fix it - it will still be there, regardless of which server you use. All you are doing is buying yourself some time before the problem reoccurs.
90% of users' problems can be resolved by punching them - the other 10% by switching off their PCs.