You guys, I'm so excited. I went ahead and upgraded a recently acquired 600XL to 64k of memory. That makes it mostly equivalent to the 800XL, only in a smaller package. The upgrade was really easy. Only needed to purchase 2 memory chips and then do a very small amount of soldering. I thought I'd post the instructions I gathered from around the web here in one document. I'd be happy to help if anyone else in interested in doing this type of upgrade.
Here's the finished product after putting it back together and cleaning it up.
And here's the free memory command that shows the same value as a 64k 800XL (don't mind the fuzzy picture)