or just get a smaller, lighter battery...Optima comes to mind...save you the trouble of moving everything to the trunk and stuff.
also, Nissan Altima KA cams will give about 10 HP (why didnt Nissan put them in there in the first place???) header, intake, exhaust like all others said...pulley also. Altimas dyno about 120 to the wheels stock (5 speed) and with intake/header/exhaust get to about 145 to the wheels. a good 25 HP increase. and yes, there is dyno proof...Altimas.net, search for them.
lightweight wheels also...cant forget about unsprung weight...