IMHO, the best way to root, because it is so well documented, is here:
Universal Eris Root for Dummies!
For the best 2.2 or 2.3 ROM, I really think that you should get GSB (
here). Just note that, unlike many flashes, it requires the download and flash of a google apps package as well as the ROM. So, you go through all of the steps to flash (Nandroid backup, wipe data, wipe dalvik, flash GSB) and then, before you restart, flash the google apps .zip file.
GSB is very stable, has great performance and battery life, and has no bugs that I know of. The only thing I'd recommend after flashing is to go to settings->CyanogenMod settings->Performance and turn on "lock home in memory."
If you want the absolutely most stable Eris ROM, look at
xtrSENSE (which is a 2.1 based Sense ROM, very close to stock, though overclocked and with a feature called cache2cache, which moves third party Dalvik cache out of the area where apps are stored and gives 20 MB or so extra space for use apps.)