This is a good place to learn some more:
http://www.atariarchives.org/
An 800 would give you more memory. The 400 may be limiting.
Or you could go for the 800XL. It has 64K. A 600XL could put you in the same spot as the 400 - the 600XL has only 16K.
There's the XE line, too.
Actually I seen a 800xl I'll look into that too but what is the best of ataris computers