Currently it has 1TB regular HD. I want a SSD! If there are two hard drive slots, I'm thinking of a 120 GB SSD for Windows/programs + using my current 1 TB for storage.
But if there's only 1 HD slot, I'm thinking a 256GB SSD, and then using a portable external hard drive to store my data.
I also have a question about SSDs. How do I know if the one I choose will be compatible with my laptop? I'm considering the SAMSUNG 840 Pro Series SATA3. The part that confuses me is SATA3 - does that number matter in terms of compatibility, or should this SSD be fine for my laptop?
You have 2 hdd bays but...
The problem is that they are both filled already. Your hdd is not a 1tb; its two 500gb drives in raid0 for a total of 1tb.
You can get an ssd as part of a kit that will come with cloning software and a usb adapter and clone the raid to the ssd and then install the ssd in place of one of the drives (preferable drive 0 but not necessary) but when you boot back up you will need to enter the bios raid utility and then add the ssd as a single drive (the 1tb raid will show as broken) before you can boot into windows. Once into windows test your ssd speeds. AHCI is usually faster but often the raid drivers are just as fast. If you want to switch to ahci mode we can work on that later or you can search Toms for a post I made a couple of weeks ago detailing how to do it.