I have not done a thing with HSPL and the radio (and I wouldn't know how) so you probably don't have to worry about that. I use it all day every day. I would suggest using either version of Froyostone. I believe it's important to have a class 6 micro sd card. I have not been able to use GPS so that's my issue.
You can install and run any Market app just fine. Go to town man. Use ADW or Launcher pro. If any app does not install, just dismount the SD card, install the app, and mount the card again.
EDIT: I am really glad to have this topic here. I need help sometimes. Is there a version that anyone thinks is more stable than Froyostone? Also, another issue that must be something with me: Flash used to work fine and now it doesn't. What would cause that? I used to pull up ESPN and watch vids and now they just don't play.
Sorry for all the edits. I just realized you said you might get an HD2. If you don't have one already, I say don't get one! It's much easier to get a native Android device. Go with the Vibrant or wait for the next best thing (dang, the G2 looks awesome and the 800 mhz processor will beat the HD2 because Android is native and it's plain vanilla). To those who already have the HD2, put Android on it by all means!