what would be the best way to display a PAL TV signal on a modern HDTV in the USA?
Do HDMI upscalers do an acceptable job of getting PAL 50hz input out to a normal HDMI 1080p 60hz?
Simple answer: Yes, they all do (plus many US HDMI TVs accept 50 Hz anyway).
But you may want to check if they are able to process PAL input, as there are some (usually older) who don't. Also, there are differences in their upscaling algorithms, so while this is no big issue for a movie it might be annoying on certain consoles/games.
So it might help to try before buying if it's about a more expensive one. On the other hand, not much damage with the cheaper (<40USD) offers.