Unfortunately, your build and budget dont leave much room for any significant upgrades. However, that doesnt mean you can't do anything.
It really depends on what you mean by 'performance' - Gaming performance? Boot up speeds? Ability to do more at once? File transfer speeds?
Gaming performance: GPU upgrade - but at $100, thats not really an option. However, you could overclock your 750ti a little for a small but free FPS boost.
General performance: CPU upgrade - but again not much you can do with $100, as the only meaningful upgrades will cost way more than that. As above, you could look into overclocking but you would need a good CPU cooler.
Multitasking ability: RAM upgrade - You could do this on your budget, but I would only advise it if you have 6GB or less. For gaming and general use, anything above 6GB will be fine at the settings a 750ti would run.
Boot times / Load times in games and apps / file transfer speed: SSD upgrade - This is something you could definitely do with your budget. If youre currently using a hard drive, then an SSD would significantly decrease the amount of time it takes to boot up and shut down your PC, load up games and apps and move large files around.
So my suggestion would be to buy your self a good 250GB SSD such as:
http://www.newegg.com/Product/Product.aspx?Item=N82E16820147372
If the prospect of a performance boost in that area doesn't do anything for you, then my suggestion would be to hold off and save up until you can upgrade your GPU or CPU with something worthwhile.