I have a nike psa with a 512 MMC card in it. You need to upgrade the flash, and make sure you get the RIO version of the nike psa and NOT the phillips version. I suspect you could put a 1G card in it, but I haven't tested it myself. Do NOT get an SD card for it, only MMC works. The psa play works great, though uploading music isn't fast.

I think I remember hearing about the S series players working, but I'm not sure.