Aftermarket upper control arms allow you to set more caster. That will improve high speed stability, especially with radial tires. There's little advantage to swapping lower arms unless you're going to a coil-over setup that requires it. If you do change the upper arms, don't discard your original parts. One day you will want them to keep the car correct. They can be easily swapped back if you change your mind.

There are also tie rod ends available that reduce the bump steer by bringing the steering linkage parallel to the a-arms. They aren't parallel as designed and the wheel changes toe dramatically during bump.

http://www.scandc.com/balljoints.htm (scroll to the bottom)

I think dialing in more caster and fixing the bump-steer are worthwhile changes.

Go to premium shocks too if you have the budget.

David Pozzi has good information at http://www.pozziracing.com/

As John says, this stuff can burn through your budget pretty quickly, so defining goals is important.