Additionally there is a possibility to disable internal memory and remap it onto card (switchable via jumper), thus one can "fix" many failing XLs without even looking inside.
Also, an unexpanded 16kB 600XLs can be upgraded to 320XL without any additional work.
RAM 320XL uses banking via bits 2,3,5,6 and 4 of PORTB. As you can see this is compatible with "Atari magazine" expansion rather than a most common RAMBO or Compy shop setup, however I have to say the comaptibility with both old and new software is excellent. There can be only problem with some programs, which does use the separate Antic/CPU banking. If you want to test any particular program, let me know.
Installation is easy. 600XL users will just plug it into PBI port. For 800XL is a bit more complicated, because Atari decided to remove 5V power from the PBI connector from it. Thus 800XL users will have either to use additional power cable which will fit into joystick port or solder one wire inside atari to make the PBI port powered like on 600XL.
The estimated price is between 20-30€ + postage. There will be probably two versions. A cheaper "naked" one and the more expensive with enclosure and power cable.
The availability is unknown, but I believe I can prepare a first batch in a month or so.
The future:
RAM 320XE - external plug and play 320kB RAM upgrade for all XE machines with EXP port. The basic parametres will be the same as the RAM 320XL plus possibility to switch into 100% compatible 130XE mode with separate ANTIC/CPU banks.
RAM 576+ - Internal RAM 576kB and beyond which will require very little and simple soldering. Universal PCB for XL/XE/XEGS. Compatible with 130XE/RAMBO/COMPY/Atari magazine modes (switchable).
Gallery:
Development:


First prototype:

Here you can see one of the protopytes in 600XL on the krupkaj's pages:
RAM 320XL in 600XL 1
RAM 320XL in 600XL 2