Looks like the 4GB Memory Stick PRO Duo is back on sale! Buy.com is selling the PSP-compatible memory card for only $20 -- that's the cheapest we've ever seen it. For many, 4GB offers enough space for music, movies and games. However, we're still keeping an eye on slowly falling 8GB Memory Stick prices. If we see a good 8GB deal, we'll let you know.