A lot. You can see it all on my cardomain link on my signature, but the actuall mounting process? Well in Japan they released a Z31(first 300zx) with an RB20DET, this was called the 200ZR. The engine mounts, sump and pick up from this car all bolt an RB straight into the all model Zs (i have 260Z by the way). The gearbox will need a custom mount but it shouldnt cost too much, the best bit is that the gear shifter will come straight into the original hole.
Everything else on my car is custom, so you could do the same, or retro fit other parts from skylines etc.
If you are tossing up between RB20 and 25, bang for buck would be 25. SR20 is a better way to go for 2ltr engines (more torque and lighter engine).
this is my zed http://www.cardomain.com/ride/2524481