The truth is, you can buy any card you want. BUT, your system is old, and any of the top GPU todays (980, 970, 390, 380) will not be have the best performance in your system.
A 750Ti is a good card, doesn't requires a extra power , only with the power supplied by PCI Slot is fine.
Now, why i'm talking about PSU? Well, you need at least -trusted- 300W to run this.
Do your current PSU have this wattage? Well, hard to say. That's why i'm saying we need a photo from your psu. If this doesn't have the wattage required, you will need a new power supply.
Now, what if your power supply have 300W but its generic? can i use it? Well, yes, you can use it, but is not recommended and reason are in my previously post.
i hope my post can help you!