Can't answer the performance boost question. I finally replaced my original battery (12-in GV, purchased in February '03, so it's a Rev A) last month. I got my battery at an Apple store.
BUT -- if you want to check pricing, try cnet.com, then click on the "price comparison" link. I think that's what it's called anyway. Then you can do a search and it will find you the best price. I've purchased some accessories this way and saved considerable money off buying from, say, Apple itself. I don't know, though, whether you get a better price on the battery. They aren't cheap, that's for sure.