That model can accept 2 -1GB RAM modules (recommneded), likely the access port is underneath the display.
The maximum OS X verison it can use is 10.6.8 installed via the 10.6.3 disk you have. No 10.7 or later versions.
I advise backing your personal data folders (Music, Docs, Pic's etc) to a external drive (via drag and drop copy methods) first before doing anything, disconnect this drive while updating OS X to prevent errors.
Once the RAM is installed, (Use the Crucial.com online checker for the correct RAM specs, videos on You Tube how to install) reboot twice and the second time stick the 10.6.3 disk in and run the installer/upgrader.
If it doesn't work you will have to follow this proceedure to erase and install the entire drive then install 10.6.3 fresh.
How to erase and install Snow Leopard 10.6
Use the same user name when setting up.
Then Software update under the Apple menu to 10.6.8
Install 10.6 compatible third party software, then files from the external drive into the same named account.
Erase the external drive and use it to set up a Carbon Copy Clone (bootable), or a TimeMachine drive as backup. (not bootable)
Once 10.6 is installed and updated, the SMC firmware is changed and you will no longer can use the 10.4 or earleir boot disks on that Mac again.
Most commonly used backup methods
