Thanks for the machine information.
Does your MBP have an access door for a battery on the bottom, or is the lower case all one piece (as pictured [http://www.ifixit.com/Info/ID-your-Mac/650|here])?
If there is an access door for your battery, you have a [http://www.ifixit.com/Guide/Device/MacBook_Pro_15%22_Unibody_Late_2008_and_Early_2009|Late 2008 MBP 15" Unibody], and the maximum is 6 GB.
If there is no access door, you have a [http://www.ifixit.com/Guide/Device/MacBook_Pro_15%22_Unibody_2.53_GHz_Mid_2009|Mid 2009 2.53 GHz MBP 15" Unibody], and the maximum is 8 GB.