I have had people mistake my iMac (HDD) for a Fusion drive iMac. My iMac boots in 35 seconds, and once an app has been opened once after a bootup, and boots just as fast as launching from an SSD (due to App Caching). So you are only benefiting from saving yourself 20 seconds on bootup once every 2-3 months, and apps still open very fast. I compared my iMac with a 15" retina MBP (had a faster i7 over my i5, twice as much RAM, and the weaker GPU) and it has an SSD. It opened apps at around 1 second faster than my iMac. I really am amazed at how little difference there was.
I feel a lot of MacRumors members compare their new SSD 2013/2014 iMac with their old 2008/2009 HDD iMac. Just not a fair comparison.
OP, the HDD is great and more than good enough.
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You can't upgrade the system. You get what you bought.
If you are looking up gaming performance for the 750m, stop. Every benchmark and site I have read uses the DDR3 version of the 750m. Look at the 755m (which only comes in GDDR5) instead for a better comparison.
I think you will be fine with modern games. Here is a video of the GDDR5 750m running Battlefield 4 at 'PS4 settings':
http://youtu.be/j8U2BJN04o0