rc7-1 is the same as the normal RC7 (or should be at this point since I had updated RC7) except that it has the fix to allow OpenGL to work with a color depth higher than the desktop depth, and a change to how the is-event-true/is-event-true-delay sexps work. The sexp change is only a UI related change though, so it's not going to actually work different at a technical level. Other than that it's the exact same build as what RC7 is, but since the sexp change is experimental, it needs specific testing and is why I haven't updated the RC7 build with the rc7-1 binaries yet.
You can use it or not, doesn't really matter.

Might be a slightly obtuse question, but how well does windows build run on x64? If we don't know, we'll find out in a few days when I get mine up and running, but I thought I'd ask ahead of time to pre-determine any snags. (already aware of the potential dual-core issues.)
The 32-bit version should work fine on Win64, but actually making a 64-bit version for Windows is completely untested. The code should be there for it to work, but I can neither build for nor test on Win64 so I haven't verified that it all works. The 64-bit Linux version works perfectly though.
EDIT: Oh, and I'm re-uploading the RC7 build. I just downloaded it again and it does appear the older set of binaries (for me at least). I know I uploaded it, but I guess I packed up the wrong set of exes or something. The new one should have FRED dates of the 28th, and FS2 dates should be the 29th. Other than the sexp change in rc7-1, the updated RC7 should have all of the other fixes.