Wii memory, sensor bar, audio, DLNA, screen options... questions, questions!

"Does memory available affect performance? Will games play poorply as Wii memeory fills up with game saves and channels?"

No. The on-board flash has a section set aside for system use. You can't effect it's performance by filling up the user accessible portion.

" I have mine on the top of the TV and it's got a slight tilt back, does that matter?"

The sensor bar is just a couple of infrared LEDs. As long as the Wiimote sees them then angle doesn't matter. You'll know if there's a problem when your cursor suddenly disappears.

" Simply enabling "Dolby PLii Game" decode on my AVR seems to work."

Select PLII Movie mode instead. Game mode is intended more for use with strict stereo sources like N64 and PSone games. You also need to turn on Surround in the Wii's system settings, instead of the default Stereo. Make it a habit of checking new games for a Surround/PLII audio option as well in their settings. Games should listen to what the Wii system settings say, but some behave poorly in that regard.

"DLNA"

You can kludge DLNA in to working through the web browser with some server software on your PC, but it's not work the effort. If you want to stream video on the Wii, use Netflix or Hulu.

480p is the correct choice. 480i still renders at 480p, then throws away half of the lines for display on anything running over 30fps. At sub-30fps it just splits the image across two fields for your display to deinterlace, which is gross.

"saves"

Only system saves count. Any save on an SD card is just for backup. Big SD cards are useful for storing lots of downloaded games, as you can access them directly through the little SD Icon on your Wii menu. They aren't useful for much else.