If you don't mind used parts, you can sometimes get some good deals on ls1tech.com.
For best bang for the buck "starter mods", I'd get some LT headers, SLP Lid, E-Cutout and/or Catback, LS6 Intake, UD Pulley. You'll need those mods anyways if you ever go Heads and Cam.
A tune can always help your bolt on mods as well.
Bill