You can put as fast an accelerator as you can find in an A500, if you like, and you can get Ethernet addons as well. What you're going to run up against is the fact that the system bus is only 3.58MHz. You're not going to get the CPU to access memory any faster than that unless it's RAM directly on the accelerator; that's why accelerator RAM and later CPUs with more cache make such a big difference. And you're not going to access the Ethernet adapter any faster, either, or the video.
That's not to say you can't do those things on the Amiga; video for certain is doable, if limited, and I believe there are at least 10/100 Ethernet cards (though you might need a Zorro II expansion box.) Certainly with a nicer accelerator and onboard RAM you can get MP3 playback. But ultimately you have to remember that the A500 is good at what it was designed for, flexible enough to do a number of things it wasn't designed for pretty well, but still none too far removed from the original 1985 Amiga 1000.
