There isn't really much difference between the 3 stages other than this:
Stage 1: You compile everything
Stage 2: You compile everything but gcc, gcc-config and libgcc++ or whatever it's called
Stage 3: A very minimal system is precompiled for you and you compile all of the extras
GRP: Everything is precompiled for you. You just install binaries straight from the cd.
If you want to do the quickest install, but also want to try some different optimizations to see what they do, then do a GRP install, and then recompile it as you run. You can do a little every night if you like. I just did a stage3 install on my laptop with very conservative "-02 -march=i686 -fomit-frame-pointer" flags. I then emerged gcc-3.4.3, and set that as my new compiler, changed my flags to "-march=pentium-m -02 -fomit-frame-pointer -pipe" re-emerged it, rebuilt my kernel and modules, rebooted, and then did an emerge -ea world which is rebuilding all of the packages on my system. I just set it to continue every night.
If you DO decide to install gentoo and want to get into the whole cflags thing, don't get caught up in it all. Some people try to put in "extreme" cflags but they don't really do much at all. The customization can help in some instances (sort of like how 64 bit generally isn't better than 32 bit in most apps, but sometimes it does make a difference), but extreme cflags are a waste of time and possibly stability.
Will it make much of a difference? Well, in SSE and SSE2 apps yeah, otherwise, probably not. But do you really notice the 6 extra fps you gain by o/cing your video card? Yet you still do it...
IMO the main reason to use gentoo is for portage. It's very easy to use and I never have problems with it. A close second is the community. I've had 99/100 issues I've ever had with gentoo solved by searching their forums and the remaining one solved by posting.
Gentoo's 64 bit release is great. There is a full forum there that will help you out if you have problems, and they are amongst the first to recognize/fix issues.
