I'm currently running an NA RB25DE in my S30 and Iv'e actually considered both a carbed setup and a FI setup with ITB's.
Both of which I don't consider to be the best bang for the buck upgrades but, definatly unique and cool sounding.
However, here are the ups and downs.
Going carbs would enable you to keep most of your stock fuel systems whereas with an FI setup it would be best to upgrade your gas tank, fuel hardlines, and a bigger fuel pump. For both applications you would have to fab a custom intake manifold for carbs or an ITB setup. The last major thing for the carbed setup would be to ditch the stock direct fire ignition system and make your own ignition system and for the ITB FI setup, you would have to leave the MAF sensor and switch to a MAP system or standalone.
If your searching for more power I'd look into doing some of the old school tricks like larger disp.(RB30), headwork and maybe some bottom end work, cams, injectors, and some ECU tuning.
but that's just my 2 cents.....