An SSD is definitely worth it. If you're getting a 128GB SSD then you should replace your 1TB with 2TB seagate barracuda green (costs 99$)
Keep the 7200RPM drive, 128GB is not enough to hold an extensive gaming library (especially if your using Steam, then you cant choose where individual games are installed to) and all your programs. You will still need a fast HDD.
Though I personally would sacrifice the SSD for a better GPU, get something from the HD78XX series.