i would go sr20det, redtop. it is pretty common now and can be done quite easily. as for the rb25, there is a little more modification due to its obeisity. i still want a 240 real bad.....wanna sell it to me, ill give you 500.01? sound good.
but i would deffinitly go sr20, very fast and a boost hog. total-boost.com has them for fair prices, tho you need to buy a few other things when purchasing it from there. alot of people will usually buy the front clip to ensure they get EVERYTHING. the only problem that i know of with the swap is that sometimes the car will get pissed and say that is running hot, and the temp gauge will go through the roof, even tho the engines not hot and wiring can sometimes be difficult if your not very familiar with those types of motors.