I'm slightly confused, are you sure its a late 2009 model as standard the 13" Macbook pro has 4gb RAM as standard. I'd suggest running your system profiler to check (click 'About this Mac' then 'more info'). From there you will be able to check exactly what you need.
Nolimit is correct (if you have the right model), it uses DDR3 1066mhz RAM, but try and get "mac specific" memory as I've had problems using 'generic' stuff before. It doesn't need to cost any more though, so be careful where you buy from.