Keep in mind that the ST-E2 uses the infrared eTTL and is not that reliable, this is the reason I purchased the Cybersync transmitters/recievers. You will also need the shoe adapter to connect the pc connection to the recievers of each flash.
If you are using off camera flash, I would recommend not using a flash on camera. In order to use eTTL via 500 series flashes, it will need to be on camera. When you are using off camera flash, aren't you trying to get rid of the on-camera flash effect? The whole eTTL system works, but is very sensative and you are limited to do certain things with it.
With wireless tramsmitters/recievers, you have the versatility to roam around a much wider range and not have to worry about the infrared system communicating. The Cybersyncs are reasonably priced and do pretty much the same thing as the pocket wizards. I bought 1x transmitter and 3 recievers for ~$330. Yeah, the Chinese manufacturers have models to replicate the same effect... but key words.. Chinese Manufacturers. You will not get the customer service or anything if any of them are defective. Where as you purchase from a USA manufacturer, they stand by their products - aka Paul C. Buff products. I have used them and whenever I had any kind of question they always had the right answer there on the spot.