I have many video files. I want convert all the files to a playable format for my phone... would H264 or xvid work? I just want to convert to a hardware decode-able format to save my phone battery.
Since XviD is just an MPEG-4 type, that's a very good bet. There are probably phones that natively decode H.264 as well, but given its relative newness and Google's recent disapproval of it, it's not as likely. For more specific information, we'd need the model of your phone. You can also just Google "Droid X chipset" or "HTC Wildfire specs", for examples, and then when you find the video chip/processor just Google whether it decodes your codec of choice.
Some devices may play more formats, but these are supported by every phone.