I'm sure you already purchased, but for those just tuning in...

Definately two flahses. I have a K10D with a single 360 and I'm in love with the wireless mode. From the manual, it looks like your *ist will not support wireless via the built-in flash. However, it looks like it will support wireless if you use one of the 360s on the hot-shoe.

This opens up a world of possibilities that IMHO more than makes up for the differences between the 360 and 540.