As I was buying another batch of E6 positive in the ISO100 and 200 range I couldn't resist buying another 2 rolls of expensive provia400X. It's such a lovely thought being able to shoot chromes up to ISO800.
Provia 400 can be easily pushed two stops. The results at ISO 1600 are excellent, and I suspect it can be pushed to 3200 with no problem. On the list of things to try.