This appears to be your machine:

Apple MacBook Pro "Core 2 Duo" 2.26 13" (SD/FW) Specs

Identifiers: Mid-2009 13" - MB990LL/A - MacBookPro5,5 - A1278 - 2326*

It will take 8 GB of RAM

Here's the specs on it: http://www.everymac.com/systems/apple/ma...

Now the question becomes, "how much RAM do I really need"? Besides the cost, you significantly reduce your battery life and increase heat. Here's a nice article on how to tell what you've used and if you need it: http://macintoshhowto.com/hardware/does-...