Linux is always fun!!
However yes, it will be very frustrating. You will have to be rather tenacious to deal with the setup of gentoo on a system like that.
The advantage is that you will have a very fast system once you finish installing everything.
Don't expect this to be a one day event. I'm pretty sure it took me nearly 24 hours just to build the kernel for the first time. If X is used, it will also take a very long time to build.

What i mean in particular is, after the initial euphoria is gone, each new update for kernel, gcc, glibc, Firefox, Thunderbird, KDE etc. etc. reduced the fun factor for me to below zero after some time on a machine with only 1 GHz, even with a second crate with 700 MHz that served as distcc helper at that time.

Things changed big time once i got my hands on an overclocked i5-2500K (4-core) @ 4500 MHz - all the sudden, Gentoo is it!
In the first days, i couldn't even wait for new upgrades to arrive, just to enjoy the sick speed of that thing, haha.

I won't argue there. Updates do kinda suck on a slow machine.