Also, emulation is a poor way of experiencing a lot of games. Various glitches are introduced to most every game and framerates are a tricky beast if you have any of those post-processing effects going on. If you're looking for fidelity, the original hardware is your only sure bet.
An interesting article on the Mythbuster's site does a good job of explaining why even the most modern, high-end PCs struggle to perfectly reproduce the even SNES.
By and large, most of the ROMs that run well on emulation do so because the particular emulator you're running them on has a myriad of patches designed specifically to augment the way your PC handles sticky portions of the games. If you plan on playing anything but the "top 25s" on a givn system, emulation is going to get a little messy.
Remember, most of these classics ran on proprietary hardware, and even a boatload of RAM doesn't always mean total fidelity.
Retro Tech Select - My Youtube channel. Covers throwback consumer electronics with a focus on "vid'ya games."
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