I don't know no PIC with flash and USB. With USB you have 16C745 and 16C765 but both are OTP or uvprom. If you want USB and flash you can try to use external hardware to run USB and interface it to serial/I2C or spi ports from a flash device.
Another option is to use the IgorPlug-USB. It's a firmware implementation of USB in an Atmel AVR flash mcu:
You can't use PICs for firmware level implementation of USB, since the PICs aren't fast enough.
www.cesko.host.sk/IgorPlugUSB/IgorPlug-USB%20(AVR)_eng.htm