Replying to SSD size/raid help

Right now I have 2 500gb drives in my desktop, and a 2tb external. I also have a lot of crap, start up disk is full, and I've wanted an sad for the speed. I'm planning on backing up my files, then doing a clean install to the ssd. I want to use the ssd for system files. I also want to put the 2 500gb drives in raid 0, then use the 2tb external for backups only If I am working on making a new edit, should I keep the raw clips on the ssd, then move them to storage, or just put them on the storage drive? Also, what size ssd should I get? Doesn't need to be insane, I have about $150 budget for the drive, but I only have data 2 connectors so I don't need the fastest drive. I was looking at 240gb drives, but I don't use more than 50gb of raw footage for a project, and it seems that a lot of people just use 120gb drives.