I beg to differ... I used to run GBA emulators with few speed problems on a Pentium III 450 back in the day. I even ran most PSX on it with minor slowdowns (though Final Fantasy IX worked near perfect even speedwise). Don't forget, PS1 was only a 33.86MHz system. GBA was only 16.8MHz. From what I've seen... to get decent results you need a computer about 8-10 times the clock speed of the system in question. See the thing is... most emulator project concentrate on features and compatibility first with speed second. So, DS emulators (even though one CPU is 33MHz and the other 66MHz) will run like ass for now. Granted a 2.0GHz CPU will run games at full speed long before a 700MHz CPU will, but the 700MHz CPU will work decently eventually because there are plenty of optimizations to be done. PS1, GBA, SNES, NES, and Genesis are the systems that your PC could easily emulate. Any next-gen machines (PS2, Xbox, GC, NDS) may have some trouble. N64 is probably borderline. I hope this info helps!
And don't be dissing DOS games, I still play stuff like Jazz Jackrabbit and One Must Fall: 2097 on DOSBox. Those games were the shit!