stylin - pull the diagnostic codes before you start replacing anything. The computer is designed to let you know through the codes if bits/pieces are malfunctioning.
If things need replacing - do so, then see what you think. The stock engine's power peak occurs right in the 4500 rpm range, so it's completely normal for a stock one to feel like it's running out of steam above that, especially in the higher gears.
If you simply want more power higher in the rpm range, you'll have to alter major engine components to achieve that - exhaust systems, intake components (maf size, throttle body size, intake manifold), heads, cam, etc. Or you can add boost via a supercharger or turbocharger - the latter is more involved. But be careful - the verts are heavier than the non-verts, and you need good low end torque to move them - especially in street driving. It's possible to mod the naturally aspirated motor in a way where your peak power is much greater and occurs much higher in the rpm range, but doing so usually significantly reduces torque production in the low rpm range - which makes one even less fun to drive on the street.
If I were in your shoes, I'd take these steps. First, pull the codes and fix anything necessary to be sure the car's running right. Second - pull the stock silencer out of the inlet to the air cleaner box, and put a flat panel
K&N filter in. Third - set the initial ignition timing to about 14 degrees btdc. Fourth, have a different set of rear gears installed in the car - 3.73's are a good compromise. Fifth, put on a set of long tube headers and a free-er flowing exhaust system. Those changes should make a big difference in the way the car feels/drives without significantly harming drivability or efficiency. If you still want more umph, then you can move on to other mods.
BTW - the folks at AutoZone will usually pull the diagnostic codes for free.
