I need more than 8GB

@Brian - Nothing you can do to increase the RAM it is what it is. But you are likely hitting a different problem here which makes you think you need more RAM when you really don't!

You see, your SSD is also used for Virtual RAM! So when your SSD has too little room left it will slllooowww dooowwwnnn your system!

OK, how to fix...

The first thing here is to do a good housecleaning getting rid of any old junk you just don't need. Here is a good housecleaning app to flush-out old log & cache files Disk Doctor

You may want to consider deleting your iTunes Movie and TV shows (or your music if you have a lot). You'll need to have iMatch services enabled so you can stream them from Apple when you want them over the network.

The goal is to leave 1/4 to 1/3 of the SSD free, If you really can't get to this its time to get a bigger SSD. Here's one source: Transcend JetDrive 725.

Lastly, I would also recommend you get an external drive to hold anything big and/or important (copy). If you can't swing the upgrading of your SSD right now. Besides if you set it up as a bootable drive you can use it to help maintain your internal drive with Disk Utility.