Here is a link to a hardware projects page, that includes Atari upgrades that can be purchased, as well as instructions on making some of your own modifications from scratch, such as Dual Pokey Stereo mod, 1200XL PBI port, and even making your own 1050 US Doubler upgrade, and lots more.
I stumbled across this page while I was looking for a do-it-yourself Dual Pokey stereo upgrade. I have decided to do-it-myself piggyback Pokey upgrade in my 1200XL, because the dual Pokey board I have will not fit in the 1200XL along side the 32-in-1 OS board. previously I had moved the stereo Pokey board to the rear where the RF modulator use to be, and used a ribbon cable to connect to the original socket for the Pokey up front. This has been problematic for me when I did it because I didn't have proper socket headers, male and female for properly attaching the Pokey board. I have had nothing but trouble trying to track down the parts needed to do it properly, so I have decided to save my Dual Pokey Board for an 800XL I am upgrading as a gift to a friend, and am instead going to do the piggyback Pokey mod in my own 1200XL.