Class 4 4GB but the OS performs decently. No major lags whatsoever.

It's very hard to approximate since it dies on me every 20-30 minutes (playing a lot around the UI and settings) and reboots use a lot of power.

Given a more fixed activity (gaming) I have not had such issues but the device ran quite hot. I'd guess 4 to 8 hours with moderate usage... but it's a very wild guess.

EDIT: and by 100% usable I don't mean that all apps will run, only a handful, but the general OS is there.