is it a Auto or 6sp? if its the 6sp your definatly going to want to get rid of the skip shift..after that just do the basic upgrades
1. get a nice catback like mike said and if u want go with a decent set of headersthey will add some good hp and a real agressive sound
2. cold air intake. farely easy to do, you can either buy a setup or do it your self following the instructions over at LS1.com in the FAQ section.
3.there's a few tricks you can do with the EGR vavle also that will allow less air restriction in the Throttle Body.
just look around at some differnt sites..there's SOOO much you can do to these motors..just takes some know how and of coarse...money.