Before you get an extended life battery may as well take a look at some information.

Here is a thread where people are talking about using the Seidio 1750 mAh in their Evo: http://androidforums.com/accessories-evo-4g/88887-seidio-inn...

And here is an older comparison of OEM and 3rd party batteries with disappointing results: http://www.batteryboss.org/