I want to buy an SSD, but don't want to spend more money than I need to.
Actually, this isn't normally in the frame of mind when purchasing an SSD. As mentioned, with regard to games, only the load times will be faster, so if you're tired of waiting 2 minutes for a game to load, then it being on an SSD might take only 35 seconds or so, give or take, depending on your other hardware.
The main reason for an enthusiast to get an SSD is for boot times. My laptop went from a 1:40 complete boot time with an HDD to roughly 0:28 on an SSD. Outside of that, program loads are faster.
You'll never run into an issue of degrading the drive performance if you go larger. Once you start filling the SSD with more than 50% data (in some drives) the performance drops. Always go larger. An old wise man once told me - if you have to think about how much it cost, you can't afford it!