1. Assuming the L2 cache is the same, you'll see a 20-25% improvement. Assuming the 500 MHz has a larger L2 cache, you'll see an even larger improvement.
2. Not sure.
3. Apple hasn't made any batteries for these in a long time. Any "new" Apple batteries have been sitting around for years and are worthless. Buy the cheapest off-brand you can find on eBay, because all of the off brand batteries (including name brands like OWC or NewerTech or BTI) come from one of a few suppliers in China, and they're all of about the same quality.