if your plannin to keep the car for a while i guess the skys the limit. but personally id be trying to sell it off and upgrade to a manual. its such better base to work from and saves you lots of money down the track.
the trend for big power these days is going the dirty 30 route (rb30 bottom end) with either a GT30 or GT35r Garret turbo for 300-400rwkw with supporting mods.
the 2 biggest questions for you to answer at this stage is.
1: what do you want the car to do/be (drift, drag, track, cruise, VIP, ect)
2: how much money do you have.
then ppl can give you much better advice. i havnt heard many good things about the autos, but goodluck