Spotted an article on The Reg today about people fitting cheap replacement batteries in their Nokia phones... Well, given I've been offering advice to all and sundry about fitting higher capacity batteries in their 7650's to help them stay powered for longer, I figured I should make you aware of the news article, and also offer my comment on it.
http://www.theregister.co.uk/content/59/29231.html
Now, the article says that Nokia have stated that these third party battery supplies have critical safety devices left out to reduce costs. In NO WAY do I condone the use of cheap and cheerful batteries, certainly not at the expense of possibly causing untold damage to your phone, or possibly inflicting bodily harm to your goodself while using your phone! This "extra cost" that is missing from cheaper Lithium Ion cells is fitted to ensure the battery isn't exposed to short circuits and high current drain which would cause overheating of the cell.
I can only suggest that while it is perfectly acceptable to use third party batteries (higher capacity or not) for cost reasons, I must emphatically suggest you purchase your batteries from a reputable supplier, not, and I repeat, NOT from a web only sales outfit where any claims against them supplying inferior batteries may prove impossible.