What's the advantage to getting a system with a SSD drive?
1. Bootup and shutdown times.
2. Software installation and update times.
3. Loading directories and lots of little files (like waiting for that save game list), or cold-cache game data loading.
4. Etc.
What do I put on that drive? Just the OS?
Whatever. It's a question of space and money. I got a 480GB, and I can fit all the games I might want to play at any given time on it, with ~200GB to spare, at any given time. Saved games, and whole modded game install folders, get backed up to my HDD (when a 50+ hour save gets corrupted, you'll stop trusting that Steam cloud crap, and start rotating backups).
Oh, and make sure to not have any storage drives but the SSD connected, while you install the OS. Windows could enumerate the HDD first, and install the system reserved partition and bootloader there, which can turn into bad news at a later date. With no such options, it will put everything on the SSD, then you can add the other drive after the install.
Would it help game performance at all? (load times, etc...)
Will vary. Some games yes, most no, some if you're tight on RAM.
But, in general, don't spec a new PC without at least a 240GB SSD for the OS, unless it's a <=$600 one. At $100-120 for a good one every day, it will be worth the money. If you don't need more space than that, you won't even need an internal HDD.