Im Running 5 Tuners 2xPVR-500MCE + 1x PVR-150MCE
All works like a charm, Though You MUST Rename each Tuner (in the UserSettings) To A Unique Name) Otherwise you will get the Two Tuner issue....
Regards,
OhmegaStar

SilverStone Lascala LC-16M (Black) - Intel D925 MB, P4 3.4ghz, 1gb ram, 1x1.9TB SATA Raid (on MediaServer) + 1x200gb SATA Local, Pioneer 110D DVD Drive, Antech Fanless 500W PSU, Geforce 6600 Fanless, Logitech diNovo 2.0 Bluetooth Mouse & Keyboard, 2xPVR-500MCE, 1xPVR-150MCE Tuners.
+ three other mce machines of various (also home-built) types