Need to know what year MBP 15 you have. Look on the About This Mac screen and More Info. Generally speaking, you should be able to upgrade that thing to at lest 8GB and it can be done by the end user (again, depending on the model). You won't be able to use the existing memory since there's only a maximum of 2 slots in the laptop, so you'll have to take out what's in there now and install 2 new modules of whatever equal size you want to give it the maximum supported amount.
Welcome to the Apple Support Communities
To know how much memory you can add, you have to know the generation of the MacBook. See > http://support.apple.com/kb/ht4132 After knowing the generation, just go to this page and select your Mac model to know the maximum memory you can add > http://eshop.macsales.com/shop/memory/Apple_MacBook_MacBook_Pro/Upgrade
You don't void the warranty installing memory (you can modify the hard drive and the memory without voiding your warranty). See > http://support.apple.com/kb/ht1270
You can do it yourself. If it is a late 2008 to 2010 MBP you can install up to 8 GB RAM. 2011 or later MBPs can accept 16 GB RAM.
The only exception is that a 2010 13" MBP can accept 16 GB RAM. Make certain that you adhere to the specific RAM specifications for your MBP. The best vendors of Mac compatible RAM are OWC and Crucial.
Model Name: MacBook Pro
Model Identifier: MacBookPro6,2
Processor Name: Intel Core i7
Processor Speed: 2.66 GHz
Number Of Processors: 1
Total Number Of Cores: 2
L2 Cache (per core): 256 KB
L3 Cache: 4 MB
Memory: 4 GB
Processor Interconnect Speed: 4.8 GT/s
Boot ROM Version: MBP61.0057.B0C
SMC Version (system): 1.58f17
Serial Number (system): W80393ZTAGZ
Hardware UUID: 3B544F8A-9528-5704-9625-3CE7F71F463D
Sudden Motion Sensor: