1. search the forum, these questions are answered in detail in different threads.
2. I love Seidio Innocase II, screen protector, and universal holster (found one that fits perfectly)
3. I bought a cheap car mount from ebay that works perfectly (vent mounted). I vent mount because in the summer the air conditioning with help keep the battery from heating too much and it keeps it out of as much sun. It cost me under $10 including shipping.
4. I would wait until froyo to buy an extended battery. See if you need it. Being a lithium battery, you can charge anytime you are near a plug or usb port. I bought 2 1500 eris replacements with battery charger (charges battery outside your phone) and it works really well. when I have heavy use and no charger, I can switch out batteries.
5. That is all the accessories I have bought. The phone doesnt need much, just protection!!!!!!