What are the best upgrade possibilities for an imac 21.5 late 2009?

I updated my software to OS X 10.10.2, and I want to upgrade my hardware.

iMac (21.5-inch, Late 2009)

Memory 4 GB 1067 MHz DDR3

Sata HDD 500GB

I'd like to have a SSD for my system and a HDD for my files. Is it possible to have both inside the mac? Wich SSD do I need to buy? Where do I need to install it? How much ram do i need? I want to speed up my system.

Kind regards