General rule of thumb is that you can get by with a 128GB SSD if you have a second hard drive and are prepared to do a bit of a tidy-up from time to time.
240-256GB SSD is the price-capacity sweet spot at the moment IMHO. Prices are close enough to the 120-128GB models and it gives you plenty of space for your OS, the usual few programs (Office and other bits and pieces) without having to worry.
Any of the following can start to run you out of space on a 250ish SSD:
- games (but you said you don't have any)
- a decent music library
- a large photo library
- even a small/modest video library
While you can store the above on a larger SSD and that has the simplicity benefit of having just the single drive, it is much cheaper to buy a separate HDD (or re-purpose the 500GB HDD you have at the moment) to store any of the above. Video/photo/music libraries rarely push a HDD speed-wise, so there's very little benefit wasting your costly SSD gigabytes on such data anyway... unless you care about the simplicity factor.
I'd suggest getting a 240-256GB SSD, installing your OS and programs on it, and then use your HDD for any music, photos and videos.