I over clocked at 520 for a long time and now bumped it up to 624 some time back and have never had ANY adverse side effects. I use Pocket Hack Master BTW in case anyone wants to know.
If you got the phone off ebay and it has a high pitch ringing sound in the earpiece, then thats likely why it was on ebay. I would try to get with the seller and get my money back unless you have insurance on your acct already and can wait a few days and then file a claim.
You do not have to call in to the carrier to file a claim IF you have insurance on the acct already. Simply go to
http://phoneclaim.com and file it online and not have to talk to or explain anything to anyone.