I used the Eris Master Application to root and I installed the Eris Official ROM.
Congrats, and enjoy.
Say I want to install another ROM. How do I go about doing that?
1) Download the ROM of your choice to the root folder of your SD card. Don't rename it, don't unzip it. You can either use the phone to download it, or transfer it from your PC. The fastest way would be to download it with a PC, and put it on the SD card using a media reader/writer attached to the PC. That way you don't even need to boot into or configure the EMA-installed ROM that you are going to throw away.
2) Power up the phone into Amon_RA's recovery boot by holding the Vol-up + End/Power buttons at the same time (until you see the screen light up).
3) Scroll down to the "Wipe" sub-menu with the trackball and select it by pressing the trackball. You'll get a new menu here; repeat the select and click action for both Wipe Data/Factory Reset and Wipe Dalvik cache. For each of these options it will ask you to confirm with yet another trackball click (or use a volume button to cancel) After you have performed both wipes, click on one of the volume buttons (read the screen for directions) to return to the top-level menu.
4) Use the trackball to scroll down to the "Install ??? from the SD card" option (sorry, again I can't remember the exact text); click the trackball once, and it will display the names of all the zip files in the root of your SD card; scroll once more (w/ trackball) to the correct .zip file (probably there will be only one, but who knows), and click with the trackball to select it. Click one more time with the trackball or use the volume keys to cancel.
5) Reboot and configure your new ROM.
Oh, and how do I overclock?
First, pick a ROM with OC' enabled and working correctly. It will have the OC control app (SetCPU) pre-installed. You'll need to do a little reading and playing around with that app - it has some subtle behaviors. I strongly suggest you read zanfur's
ENTIRE post on XDA. It's a long post, but it explains exactly what is going on and what the risks are.
eu1