Hello Doug, and welcome to the Mindboards forums!
The RIS sets used studded technic parts, and the NXT sets use studless technic parts. Both types of beams play nicely together, and all gears, pins, axles, bushings etc. are identical (i.e. they are compatible, not necessarily to say that the NXT sets includes the same parts or quantities as the RIS sets).
All RCX sensors and motors are 100% compatible with the NXT, but you will need a converter cable
(costs $10 for a pack of 3).
If you want to make the NXT and RCX talk together, then you will need an IR Link
(costs $55). Note however, that communication is very slow, and not ideal for real-time control.
Just a suggestion, but I highly recommend you program the NXT using Not eXactly C (NXC, which is based on the C programming language). It's much more flexible than the retail NXT-G programming language. The NXC firmware (basically the OS that the NXT runs) is 100% backwards compatible with NXT-G, so you can use either programming language without needing to change FW. The easiest way to use NXC, is to install the BricxCC
IDE (which includes the NXC and NBC compilers, and takes care of all the dirty work of compiler linking etc.).