Well, you need to install a custom ROM or Kernal to take full advantage of the device. You'll need to head over to the Epic 4G Touch Development forums on XDA and look for a ROM to use. There are a few good ones listed in the root section here but you'll be redirected to XDA for more resources.
In terms of apps, I'll suggest Titanium Backup Pro (or MyBackup Pro) to keep backups of apps and data for future use. You can also use WiFi tether apps now that you have root access. Custom ROMs have the Sprint Hotspot app available for tether if you use a ROM.
Good luck!
Edit: Be sure to read up on the install instructions before attempting to flash ROMs. If you decide to try and of the ICS ROMs be sure to understand how you need to install those and what you need to do to revert back to a GB ROM.