*Drooling* 8-)...

Moonsound? I've heard some demos and they are great but I'm not quite in the Z80-age - I'll keep that for my future retro years!
The soon-to-be-released "VSU" with six (as in "6") graphics chips seems to be a bit more interesting, though ;-)

800XL and expand it as you wish. Though it doesn't have 128KB RAM - like the 130XE - it can be expanded to a full MB (not for the feeble minded person). 128KB is very useful as the newer DOS variants load themselves into this memory and establish a Ramdisk there.
There are few programs that use more than 128KB, AFAIK, and no commercial games.
Sadly, the 130XE has a worse keyboard than the XL generation and all other machines are to be avoided, IMHO (either only 16KB RAM, less outputs etc.).

The only other machine I would consider is the original 800 - it can be expanded with modern cartridges to more than the standard 48KB RAM and its keyboard is also good. It has no external parallel port and relies on the serial I/O connector but this is usually no problem - there are even SD-card readers functioning as hard drive replacements now that can load disk images. Some people say it has the best video quality but I can't confirm this. The 800 is much more expensive, though, but it has one advantage: It gives the "original Atari feeling" and it has four joystick ports which all later models lack.

More:
http://en.wikipedia.org/wiki/Atari_8-bit_family

take care,
Calibrator