PAL Falcon with the latest XBReboot and XeXLoader out there atm.
So far I've only tried The Saboteur off of an external 7200rpm USB HDD formatted with FAT32. The game will freeze at pretty much random points (in game and loading screens) for a minute or two and then resume again, and any buttons I've pressed meanwhile will all happen at once.
Gonna try a few other games and maybe also try formatting the drive with HFS+ since I hear FAT32 has huge inefficiencies when it comes to partitions bigger than 32GB.
Also planning on adding an eSATA port to let me use any 3.5" HDD in a nice enclosure, but that's a different story and will take a while to get together.