If you really want to do it I'd be looking for a "06-07" STI front cut with matching rear end with 180 diff & Brembo's so you end up with a good starting base I.E. 6 speed, twin scroll, Brembo's, fairly strong engine then as you are destroying this motor you can be having a stronger one being built............ but it's a never ending money pit if you want to keep going faster, I'd say just to do the swap with the later STI would cost you 16-18k whereas if you keep the RX as a runner and buy something like Type25's above which is already a beast you'd be much better off, you just don't get the pleasure of having a sleeper that you can park anywhere without worrying about it being vandalised like a nice WRX or scare the living shit out of people who think you are just driving a noisy old dunger
Supercharger...........not much room and costly, a twin scroll will get you off the line pretty quick and a 150hp shot of Nitrous is much cheaper too
Edit, sorry make that 06-07 STI not 97/98, that would be quite a bit cheaper.