I don't really know much about what you're doing with HD, but maybe try xubuntu for that low end pc.
But I don't know if xubuntu would support that or if Gentoo is faster.
I'm streaming HD video from my Suse Myth box. Because my box is slow, it stutters on playback just a bit.
Xubuntu wouldn't help. Gentoo allows you to compile a kernel for your specific CPU to extract the last tiny bit of performance out of it. You can fully optimize your kernel for the exact CPU and chipset you have.