Why not just do the 64k hack? Easy as pie.
http://www.atariage.com/forums/topic/183694-600xl-64k-upgrade-pictures/
agreed that the internal RAM upgrade is easy to do, and you don't increase the cute 600's footprint. but if you are still looking for a 1064 Best Electronics have them NEW for $50 but your 600xl will take up more space than an 800xl!Why not just do the 64k hack? Easy as pie.
http://www.atariage.com/forums/topic/183694-600xl-64k-upgrade-pictures/
Thanks now I have another option I just learning atari computers thanks
for the info TJ..