BlackBerry OS 6 compatible with Curve 8520?

THe 8520 would not be compatible with BBOS6. It would be very slow and it's just not smart. There isn't enough processing power and memory for BBOS6. You're better off getting the 9700.You can update the firmware, but you would have to do it through leaked firmware releases. You can find more information on that at the Crackberry forums or blackberryos.com. For example, I have a Blackberry 8900. Normal stock firmware would be the BBOS4, but I'm using the BBOS5 that's currently being used on the 9700. There is upgradability, but your phone has to be able to handle the new firmware.