Why not a 1TB SSD?
Can you afford it?
The Crucial 512GB SSD is around $400, if you can afford it, go ahead.
You can get a 256GB Samsung 830 for ~$200. Why not save the extra 200?
Usually anything passed 200GB on your machine is going to be data that won't really benefit from SSDs, such as MP3s, videos, etc.
If you do editing, SSDs will definitely come in handy. But if you're just playing back stuff, it won't.
I say save the $, get 256GB and put your 500GB in the optical drive, if you really need the extra 500GB. Very soon 512GB SSDs will hit the $200 mark and you can just get that and keep your current SSD and put it in another machine 🙂