Well, a 8320 is WIFI only and is a GSM phone and runs the 4.2 OS build. (capable of running 4.5 but thats another story and it hasn't been released yet)
A 8330 is WIFI and GPS and is a CDMA version. Also the 8330 comes with the 4.3 build OS which enables a few extra features. Video recording, supports a larger SD card etc.

Forgive me if I'm wrong but i do not believe the 8320 will work on Verizon seeing its a CDMA network.

The performance aspect shouldn't really change no matter what Curve model you choose. They are all RIM devices, and are usually flawless, imo.

I would assume the 8330 is a better phone. More features, and a army of people whom follow you around asking "Can you hear me now?"

Good luck.