I recommend you not update to the 4.3 bootloader as the 4.3 bootloader can cause problems when trying to flash back to stock. Once you're on a 4.3 bootloader, you cannot downgrade to a lower version. Most ROMs seem to work fine with a 4.1.1 or 4.1.2 bootloader.
If you have the att s3 (i747) and not the Bell/Telus/Rogers s3 (i747m), you cannot use Odin to flash a stock 4.3 ROM as it does not exist. You would need to flash a version of 4.3 that has been custom compiled for flashing via Odin.
I would download the latest version of Philz Touch and flash the tar version via Odin. Copy a custom ROM to your SD card, boot to recovery, wipe cache, Dalvik, data, and flash the ROM.