HELP with performance upgrade

I have a 2000 Chevy Silverado 5.3L
I am looking up do some upgrading and need some advice.  So far I have invested in a K&N air intake kit and I have a flowmaster muffler but still use stock exhaust.  Right now I am leaning towards a flowmastercat back exhaust system and dynatech headers/y-pipe/hi-flow cats package and possibly a computer tune up.  What's your take on the best upgrade for the money, supercharger, turbocharger, or exhaust upgrade?  

I have same truck, 2000 with 5.3L, Extend cab sportside.  When I bought it 2 years ago it came with a Flowmaster cat-back dual exhaust.  I don't know when it was put on but on tip fell off about a month ago. When I inspected the system all the welds are rotted and the mufflers guts are coming apart, rattling sound when starting and when I tap on the muffler.  I just bought a Magnaflow cat-back.  I will put on this weekend.  It looks like that there is not a gasket at the flange in front of the muffler.  I checked w/GM parts at a local dealership.  Parts breakdown doesn't show one!?!