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What model MacBook Pro?

How to identify MacBook Pro models - Apple Support

Based on the limited info you supplied it could be a mid 2009

MacBookPro5,1(15")MacBookPro5,2(17")MacBookPro5,3(15")MacBookPro5,4(15")MacBookPro5,5(13")

Those can go to EC and can be upgraded to 8 GB of memory (2x 4 GB)

http://eshop.macsales.com/shop/memory/Apple_MacBook_MacBook_Pro/Upgrade/DDR3

Hi jon sun:

A good place to look for upgrading your 13" Mid 2009 Macbook Pro is OWC.

Check out the following link: https://eshop.macsales.com/shop/memory/Apple_MacBook_MacBook_Pro/Upgrade/DDR3

If you would like to upgrade to El Capitan it would be a good idea for you to upgrade your memory to at least 8GB.

Once you have upgraded your memory just go to the mac app store and download the latest version of El Capitan 10.11.3.

OWC also has a number of upgrade videos you may want to check out.

If you really want to increase the speed of your macbook pro, you could also think about upgrading to an SSD drive.

To upgrade you first have to update to 10.6.8. To do that y have to purchase the Snow Leopard 10.6.3 DVD

http://store.apple.com/us/product/MC573/mac-os-x-106-snow-leopard

Then you have to fully update to 10.6.8 to do this

http://www.apple.com/osx/how-to-upgrade/?cid=wwa-us-kwg-mac

Note that some apps may not work is EC

Upgrading to 10.7 and above, don't forget Rosetta!