Comment by satiated_grue

There are a great many modern modifications available for the 8-bit Ataris, many of them from Poland where the machines saw good sales quite late compared to the US.

Using modern electronics (FPGAs etc.), processors, and high-density memories, you can imagine the processing, graphics, and I/O improvements that can be made for relatively low cost.

Many hobbyist machines at this point are highly modified, with much new software taking advantage of the new features, so specifying "classic unmodified" pretty much means a system into which you could have slapped a ROM cartridge purchased at your local computer store back in the day. XL/XE sounds like it rules out the original 800 and 400 models.