Once you have earned €46,600 (2006) or €48,800 (2007) you stop paying 4% PRSI on your income. You still pay 2% on all income (and more if you earn more than €100K in 2007). Obviously you are better off if you pay less deductions. If you hit the PRSI limit then you should notice a difference (increase) in your take home pay and a reduction in PRSI deductions on your payslips.
In the case above (allowing for the slight rounding of the figures) I believe that you only get 2% health levy relief on pension contributions above the employee PRSI contribution limit - i.e. you don't also get 4% PRSI relief on those contributions. But the thread above should (eventually) explain all. I think.